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=======================================================
Generating Python Bytecode with ``peak.util.assembler``
=======================================================

``peak.util.assembler`` is a simple bytecode assembler module that handles most
low-level bytecode generation details like jump offsets, stack size tracking,
line number table generation, constant and variable name index tracking, etc.
That way, you can focus your attention on the desired semantics of your
bytecode instead of on these mechanical issues.

In addition to a low-level opcode-oriented API for directly generating specific
bytecodes, this module also offers an extensible mini-AST framework for
generating code from high-level specifications.  This framework does most of
the work needed to transform tree-like structures into linear bytecode
instructions, and includes the ability to do compile-time constant folding.

Changes since version 0.0.1:

* Added massive quantities of new documentation and examples

* Full block, loop, and closure support

* High-level functional code generation from trees, with smart labels and
  blocks, constant folding, extensibility, smart local variable names, etc.

* The ``.label()`` method was renamed to ``.here()`` to distinguish it from
  the new smart ``Label`` objects.

* Docs and tests were moved to README.txt instead of assembler.txt

* Added a demo that implements a "switch"-like statement template that shows
  how to extend the code generation system and how to abuse ``END_FINALLY``
  to implement a "computed goto" in bytecode.

* Various bug fixes

There are a few features that aren't tested yet, and not all opcodes may be
fully supported.  Notably, the following features are still NOT reliably
supported yet:

* Wide jump addressing (for generated bytecode>64K in size)

* The ``dis()`` module in Python 2.3 has a bug that makes it show incorrect
  line numbers when the difference between two adjacent line numbers is
  greater than 255.  This causes two shallow failures in the current test
  suite when it's run under Python 2.3.

If you find any other issues, please let me know.

Please also keep in mind that this is a work in progress, and the API may
change if I come up with a better way to do something.

Questions and discussion regarding this software should be directed to the
`PEAK Mailing List <http://www.eby-sarna.com/mailman/listinfo/peak>`_.

.. contents:: **Table of Contents**


--------------
Programmer API
--------------


Code Objects
============

To generate bytecode, you create a ``Code`` instance and perform operations
on it.  For example, here we create a ``Code`` object representing lines
15 and 16 of some input source::

    >>> from peak.util.assembler import Code
    >>> c = Code()
    >>> c.set_lineno(15)   # set the current line number (optional)
    >>> c.LOAD_CONST(42)

    >>> c.set_lineno(16)   # set it as many times as you like
    >>> c.RETURN_VALUE()

You'll notice that most ``Code`` methods are named for a CPython bytecode
operation, but there also some other methods like ``.set_lineno()`` to let you
set the current line number.  There's also a ``.code()`` method that returns
a Python code object, representing the current state of the ``Code`` you've
generated::

    >>> from dis import dis
    >>> dis(c.code())
      15          0 LOAD_CONST               1 (42)
      16          3 RETURN_VALUE

As you can see, ``Code`` instances automatically generate a line number table
that maps each ``set_lineno()`` to the corresponding position in the bytecode.

And of course, the resulting code objects can be run with ``eval()`` or
``exec``, or used with ``new.function`` to create a function::

    >>> eval(c.code())
    42

    >>> exec c.code()   # exec discards the return value, so no output here

    >>> import new
    >>> f = new.function(c.code(), globals())
    >>> f()
    42


Opcodes and Arguments
=====================

``Code`` objects have methods for all of CPython's symbolic opcodes.  Generally
speaking, each method accepts either zero or one argument, depending on whether
the opcode accepts an argument.

Python bytecode always encodes opcode arguments as 16 or 32-bit integers, but
sometimes these numbers are actually offsets into a sequence of names or
constants.  ``Code`` objects take care of maintaining these sequences for you,
allowing you to just pass in a name or value directly, instead of needing to
keep track of what numbers map to what names or values.

The name or value you pass in to such methods will be looked up in the
appropriate table (see `Code Attributes`_ below for a list), and if not found,
it will be added::

    >>> c = Code()
    >>> c.co_consts, c.co_varnames, c.co_names
    ([None], [], [])

    >>> c.LOAD_CONST(42)
    >>> c.LOAD_FAST('x')
    >>> c.LOAD_GLOBAL('y')
    >>> c.LOAD_NAME('z')

    >>> c.co_consts, c.co_varnames, c.co_names
    ([None, 42], ['x'], ['y', 'z'])

The one exception to this automatic addition feature is that opcodes referring
to "free" or "cell" variables will not automatically add new names, because the
names need to be defined first::

    >>> c.LOAD_DEREF('q')
    Traceback (most recent call last):
      ...
    NameError: ('Undefined free or cell var', 'q')

In general, opcode methods take the same arguments as their Python bytecode
equivalent.  But there are a few special cases.


Call Arguments
--------------

First, the ``CALL_FUNCTION()``, ``CALL_FUNCTION_VAR()``, ``CALL_FUNCTION_KW()``,
and ``CALL_FUNCTION_VAR_KW()`` methods all take *two* arguments, both of which
are optional.  (The ``_VAR`` and ``_KW`` suffixes in the method names indicate
whether or not a ``*args`` or ``**kwargs`` or both are also present on the
stack, in addition to the explicit positional and keyword arguments.)

The first argument of each of these methods, is the number of positional
arguments on the stack, and the second is the number of keyword/value pairs on
the stack (to be used as keyword arguments).  Both default to zero if not
supplied::

    >>> c = Code()
    >>> c.LOAD_CONST(type)
    >>> c.LOAD_CONST(27)
    >>> c.CALL_FUNCTION(1)      # 1 positional, no keywords
    >>> c.RETURN_VALUE()

    >>> eval(c.code())          # computes type(27)
    <type 'int'>

    >>> c = Code()
    >>> c.LOAD_CONST(dict)
    >>> c.LOAD_CONST('x')
    >>> c.LOAD_CONST(42)
    >>> c.CALL_FUNCTION(0,1)    # no positional, 1 keyword
    >>> c.RETURN_VALUE()

    >>> eval(c.code())          # computes dict(x=42)
    {'x': 42}


Jump Targets
------------

Opcodes that perform jumps or refer to addresses can be invoked in one of
two ways.  First, if you are jumping backwards (e.g. with ``JUMP_ABSOLUTE`` or
``CONTINUE_LOOP``), you can obtain the target bytecode offset using the
``.here()`` method, and then later pass that offset into the appropriate
method::

    >>> c = Code()
    >>> where = c.here()         # get a location at the start of the code

    >>> c.LOAD_CONST(42)
    >>> c.JUMP_ABSOLUTE(where)   # now jump back to it

    >>> dis(c.code())
      0     >>    0 LOAD_CONST               1 (42)
                  3 JUMP_ABSOLUTE            0

But if you are jumping *forward*, you will need to call the jump or setup
method without any arguments.  The return value will be a "forward reference"
object that can be called later to indicate that the desired jump target has
been reached::

    >>> c = Code()
    >>> forward = c.JUMP_ABSOLUTE() # create a jump and a forward reference

    >>> c.LOAD_CONST(42)            # this is what we want to skip over

    >>> forward()   # calling the reference changes the jump to point here
    >>> c.LOAD_CONST(23)
    >>> c.RETURN_VALUE()

    >>> dis(c.code())
      0           0 JUMP_ABSOLUTE            6
                  3 LOAD_CONST               1 (42)
            >>    6 LOAD_CONST               2 (23)
                  9 RETURN_VALUE

    >>> eval(c.code())
    23


Other Special Opcodes
---------------------

The ``MAKE_CLOSURE`` method takes an argument for the number of default values
on the stack, just like the "real" Python opcode.  However, it also has an
an additional required argument: the number of closure cells on the stack.
The Python interpreter normally gets this number from a code object that's on
the stack, but ``Code`` objects need this value in order to update the
current stack size, for purposes of computing the required total stack size::

    >>> def x(a,b):     # a simple closure example
    ...     def y():
    ...         return a+b
    ...     return y

    >>> c = Code()
    >>> c.co_cellvars = ('a','b')

    >>> c.LOAD_CLOSURE('a')
    >>> c.LOAD_CLOSURE('b')
    >>> c.LOAD_CONST(None)  # in real code, this'd be a Python code constant
    >>> c.MAKE_CLOSURE(0,2) # no defaults, 2 free vars in the new function


High-Level Code Generation
==========================

Typical real-life code generation use cases call for transforming tree-like
data structures into bytecode, rather than linearly outputting instructions.
``Code`` objects provide for this using a simple but high-level transformation
API.

``Code`` objects may be *called*, passing in one or more arguments.  Each
argument will have bytecode generated for it, according to its type:


Simple Constants
----------------

If an argument is an integer, long, float, complex, string, unicode, boolean,
``None``, or Python code object, it is treated as though it was passed to
the ``LOAD_CONST`` method directly::

    >>> c = Code()
    >>> c(1, 2L, 3.0, 4j+5, "6", u"7", False, None, c.code())
    >>> dis(c.code())
      0           0 LOAD_CONST               1 (1)
                  3 LOAD_CONST               2 (2L)
                  6 LOAD_CONST               3 (3.0)
                  9 LOAD_CONST               4 ((5+4j))
                 12 LOAD_CONST               5 ('6')
                 15 LOAD_CONST               6 (u'7')
                 18 LOAD_CONST               7 (False)
                 21 LOAD_CONST               0 (None)
                 24 LOAD_CONST               8 (<code object <lambda> at ...>)


Simple Containers
-----------------

If an argument is a tuple, list, or dictionary, code is generated to
reconstruct the given data, recursively::

    >>> c = Code()
    >>> c({1:(2,"3"), 4:[5,6]})
    >>> dis(c.code())
      0           0 BUILD_MAP                0
                  3 DUP_TOP
                  4 LOAD_CONST               1 (1)
                  7 LOAD_CONST               2 (2)
                 10 LOAD_CONST               3 ('3')
                 13 BUILD_TUPLE              2
                 16 ROT_THREE
                 17 STORE_SUBSCR
                 18 DUP_TOP
                 19 LOAD_CONST               4 (4)
                 22 LOAD_CONST               5 (5)
                 25 LOAD_CONST               6 (6)
                 28 BUILD_LIST               2
                 31 ROT_THREE
                 32 STORE_SUBSCR


Arbitrary Constants
-------------------

The ``Const`` wrapper allows you to treat any object as a literal constant,
regardless of its type::

    >>> from peak.util.assembler import Const

    >>> c = Code()
    >>> c( Const( (1,2,3) ) )
    >>> dis(c.code())
      0           0 LOAD_CONST               1 ((1, 2, 3))

As you can see, the above creates code that references an actual tuple as
a constant, rather than generating code to recreate the tuple using a series of
``LOAD_CONST`` operations followed by a ``BUILD_TUPLE``.


Local and Global Names
----------------------

The ``Local`` and ``Global`` wrappers take a name, and load either a local or
global variable, respectively::

    >>> from peak.util.assembler import Global, Local

    >>> c = Code()
    >>> c( Local('x'), Global('y') )
    >>> dis(c.code())
      0           0 LOAD_FAST                0 (x)
                  3 LOAD_GLOBAL              0 (y)


As with simple constants and ``Const`` wrappers, these objects can be used to
construct more complex expressions, like ``{a:(b,c)}``::

    >>> c = Code()
    >>> c( {Local('a'): (Local('b'), Local('c'))} )
    >>> dis(c.code())
      0           0 BUILD_MAP                0
                  3 DUP_TOP
                  4 LOAD_FAST                0 (a)
                  7 LOAD_FAST                1 (b)
                 10 LOAD_FAST                2 (c)
                 13 BUILD_TUPLE              2
                 16 ROT_THREE
                 17 STORE_SUBSCR

If the code object is not using "fast locals" (i.e. ``CO_OPTIMIZED`` isn't
set), local variables will be dereferenced using ``LOAD_NAME`` instead of
``LOAD_FAST``, and if the referenced local name is a "cell" or "free"
variable, ``LOAD_DEREF`` is used instead::

    >>> from peak.util.assembler import CO_OPTIMIZED
    >>> c = Code()
    >>> c.co_flags &= ~CO_OPTIMIZED
    >>> c.co_cellvars = ('y',)
    >>> c.co_freevars = ('z',)
    >>> c( Local('x'), Local('y'), Local('z') )
    >>> dis(c.code())
      0           0 LOAD_NAME                0 (x)
                  3 LOAD_DEREF               0 (y)
                  6 LOAD_DEREF               1 (z)


Calling Functions and Methods
-----------------------------

    >>> from peak.util.assembler import Call

The ``Call`` wrapper takes 1-4 arguments: the expression to be called, a
sequence of positional arguments, a sequence of keyword/value pairs for
explicit keyword arguments, an "*" argument, and a "**" argument.  To omit any
of the optional arguments, just pass in an empty sequence in its place::

    >>> c = Code()
    >>> c( Call(Global('type'), [Const(27)]) )

    >>> dis(c.code())   # type(27)
      0           0 LOAD_GLOBAL              0 (type)
                  3 LOAD_CONST               1 (27)
                  6 CALL_FUNCTION            1

    >>> c = Code()
    >>> c(Call(Global('dict'), (), [('x', 42)]))

    >>> dis(c.code())   # dict(x=42)
      0           0 LOAD_GLOBAL              0 (dict)
                  3 LOAD_CONST               1 ('x')
                  6 LOAD_CONST               2 (42)
                  9 CALL_FUNCTION            256

    >>> c = Code()
    >>> c(Call(Global('foo'), (), (), Local('args'), Local('kw')))

    >>> dis(c.code())   # foo(*args, **kw)
      0           0 LOAD_GLOBAL              0 (foo)
                  3 LOAD_FAST                0 (args)
                  6 LOAD_FAST                1 (kw)
                  9 CALL_FUNCTION_VAR_KW     0


Returning Values
----------------

The ``Return(target)`` wrapper generates code for its target, followed by
a ``RETURN_VALUE`` opcode::

    >>> from peak.util.assembler import Return

    >>> c = Code()
    >>> c( Return(1) )
    >>> dis(c.code())
      0           0 LOAD_CONST               1 (1)
                  3 RETURN_VALUE


``Code`` objects also have a ``return_()`` method that provides a more compact
spelling of the same thing::

    >>> c = Code()
    >>> c.return_((1,2))
    >>> dis(c.code())
      0           0 LOAD_CONST               1 (1)
                  3 LOAD_CONST               2 (2)
                  6 BUILD_TUPLE              2
                  9 RETURN_VALUE

Both ``Return`` and ``return_()`` can be used with no argument, in which case
``None`` is returned::

    >>> c = Code()
    >>> c.return_()
    >>> c( Return() )
    >>> dis(c.code())
      0           0 LOAD_CONST               0 (None)
                  3 RETURN_VALUE
                  4 LOAD_CONST               0 (None)
                  7 RETURN_VALUE


Labels and Jump Targets
-----------------------

The forward reference callbacks returned by jump operations are also usable
as code generation values, indicating that the jump should go to the
current location.  For example::

    >>> c = Code()
    >>> forward = c.JUMP_FORWARD()
    >>> c( 1, 2, forward, Return(3) )
    >>> dis(c.code())
      0           0 JUMP_FORWARD             6 (to 9)
                  3 LOAD_CONST               1 (1)
                  6 LOAD_CONST               2 (2)
             >>   9 LOAD_CONST               3 (3)
                 12 RETURN_VALUE

However, there's an easier way to do the same thing, using ``Label`` objects::

    >>> from peak.util.assembler import Label
    >>> c = Code()
    >>> skip = Label()

    >>> c(skip.JUMP_FORWARD, 1, 2, skip, Return(3))
    >>> dis(c.code())
      0           0 JUMP_FORWARD             6 (to 9)
                  3 LOAD_CONST               1 (1)
                  6 LOAD_CONST               2 (2)
             >>   9 LOAD_CONST               3 (3)
                 12 RETURN_VALUE

This approach has the advantage of being easy to use in complex trees.
``Label`` objects have attributes corresponding to every opcode that uses a
bytecode address argument.  Generating code for these attributes emits the
the corresponding opcode, and generating code for the label itself defines
where the previous opcodes will jump to.  Labels can have multiple jumps
targeting them, either before or after they are defined.  But they can't be
defined more than once::

    >>> c(skip)
    Traceback (most recent call last):
      ...
    AssertionError: Label previously defined


Constant Detection and Folding
==============================

The ``const_value()`` function can be used to check if an expression tree has
a constant value, and to obtain that value.  Simple constants are returned
as-is::

    >>> from peak.util.assembler import const_value

    >>> simple_values = [1, 2L, 3.0, 4j+5, "6", u"7", False, None, c.code()]

    >>> map(const_value, simple_values)
    [1, 2L, 3.0, (5+4j), '6', u'7', False, None, <code object <lambda> ...>]

Values wrapped in a ``Const()`` are also returned as-is::

    >>> map(const_value, map(Const, simple_values))
    [1, 2L, 3.0, (5+4j), '6', u'7', False, None, <code object <lambda> ...>]

But no other node types produce constant values; instead, ``NotAConstant`` is
raised::

    >>> const_value(Local('x'))
    Traceback (most recent call last):
      ...
    NotAConstant: <bound method str.Local of 'x'>

Tuples of constants are recursively replaced by constant tuples::

    >>> const_value( (1,2) )
    (1, 2)

    >>> const_value( (1, (2, Const(3))) )
    (1, (2, 3))

But any non-constant values anywhere in the structure cause an error::

    >>> const_value( (1,Global('y')) )
    Traceback (most recent call last):
      ...
    NotAConstant: <bound method str.Global of 'y'>

As do any types not previously described here::

    >>> const_value([1,2])
    Traceback (most recent call last):
      ...
    NotAConstant: [1, 2]

Unless of course they're wrapped with ``Const``::

    >>> const_value(Const([1,2]))
    [1, 2]


The ``Call`` wrapper can also do simple constant folding, if all of its input
parameters are constants.  (Actually, the `args` and `kwargs` arguments must be
*sequences* of constants and 2-tuples of constants, respectively.)

If a ``Call`` can thus compute its value in advance, it does so, returning a
``Const`` node instead of a ``Call`` node::

    >>> Call( Const(type), [1] )
    <bound method type.Const of <type 'int'>>

Thus, you can also take the ``const_value()`` of such calls::

    >>> const_value( Call( Const(dict), [], [('x',27)] ) )
    {'x': 27}

Which means that constant folding can propagate up an AST if the result is
passed in to another ``Call``::

    >>> Call(Const(type), [Call( Const(dict), [], [('x',27)] )])
    <bound method type.Const of <type 'dict'>>

Notice that this folding takes place eagerly, during AST construction.  If you
want to implement delayed folding after constant propagation or variable
substitution, you'll need to recreate the tree, or use your own custom AST
types.  (See `Custom Code Generation`_, below.)

Note that you can disable folding using the ``fold=False`` keyword argument to
``Call``, if you want to ensure that even compile-time constants are computed
at runtime.  Compare::

    >>> c = Code()
    >>> c( Call(Const(type), [1]) )
    >>> dis(c.code())
      0           0 LOAD_CONST               1 (<type 'int'>)

    >>> c = Code()
    >>> c( Call(Const(type), [1], fold=False) )
    >>> dis(c.code())
      0           0 LOAD_CONST               1 (<type 'type'>)
                  3 LOAD_CONST               2 (1)
                  6 CALL_FUNCTION            1

Folding is also *automatically* disabled for calls with no arguments of any
kind (such as ``globals()`` or ``locals()``), whose values are much more likely
to change dynamically at runtime::

    >>> c = Code()
    >>> c( Call(Const(locals)) )
    >>> dis(c.code())
      0           0 LOAD_CONST               1 (<built-in function locals>)
                  3 CALL_FUNCTION            0

Note, however, that folding is disabled for *any* zero-argument call,
regardless of the thing being called.  It is not specific to ``locals()`` and
``globals()``, in other words.


Custom Code Generation
======================

Code generation is extensible: you can use any callable as a code-generation
target.  It will be called with exactly one argument: the code object.  It can
then perform whatever operations are desired.

In the most trivial case, you can use any unbound ``Code`` method as a code
generation target, e.g.::

    >>> c = Code()
    >>> c.LOAD_GLOBAL('foo')
    >>> c(Call(Code.DUP_TOP, ()))
    >>> dis(c.code())
      0           0 LOAD_GLOBAL              0 (foo)
                  3 DUP_TOP
                  4 CALL_FUNCTION            0

As you can see, the ``Code.DUP_TOP()`` is called on the code instance, causing
a ``DUP_TOP`` opcode to be output.  This is sometimes a handy trick for
accessing values that are already on the stack.  More commonly, however, you'll
want to implement more sophisticated callables, perhaps something like::

    >>> from peak.util.assembler import ast_curry

    >>> def TryFinally(block1, block2, code=None):
    ...     if code is None:
    ...         return ast_curry(TryFinally, block1, block2)
    ...     code(
    ...         Code.SETUP_FINALLY,
    ...             block1,
    ...         Code.POP_BLOCK,
    ...             block2,
    ...         Code.END_FINALLY
    ...     )

    >>> def ExprStmt(value, code=None):
    ...     if code is None:
    ...         return ast_curry(ExprStmt, value)
    ...     code( value, Code.POP_TOP )

    >>> c = Code()
    >>> c( TryFinally(ExprStmt(1), ExprStmt(2)) )
    >>> dis(c.code())
      0           0 SETUP_FINALLY            8 (to 11)
                  3 LOAD_CONST               1 (1)
                  6 POP_TOP
                  7 POP_BLOCK
                  8 LOAD_CONST               0 (None)
            >>   11 LOAD_CONST               2 (2)
                 14 POP_TOP
                 15 END_FINALLY

The ``ast_curry()`` utility function returns an ``instancemethod`` chain that
binds the given arguments to the given function, creating a hashable and
comparable data structure -- a trivial sort of "AST node".  Just follow the
code pattern above, using a ``code=None`` final argument, and returning a
curried version of the function if ``code is None``.  Otherwise, your function
should simply do whatever is needed to "generate" the arguments.

(This is exactly the same pattern that ``peak.util.assembler`` uses internally
to implement ``Const``, ``Call``, ``Local``, and other wrapper functions.)

The ``ast_curry()`` utility function isn't quite perfect; due to a quirk of the
``instancemethod`` type, it can't save arguments whose value is ``None``: if
you pass a ``None`` argument to ``ast_curry()``, it will be replaced with a
special ``nil`` object that tests as false, and generates a ``None`` constant
when code is generated for it.  If your function accepts any arguments that
might have a value of ``None``, you must correctly handle the cases where you
receive a value of ``nil`` (found in ``peak.util.assembler``) instead of
``None``.

However, if you can use ``ast_curry()`` to generate your AST nodes, you will
have objects that are hashable and comparable by default, as long as none of
your child nodes are unhashable or incomparable.  This can be useful for
algorithms that require comparing AST subtrees, such as common subexpression
elimination.

If you want to incorporate constant-folding into your AST nodes, you can do
so by checking for constant values and folding them at either construction
or code generation time.  For example, this ``And`` node type folds constants
during code generation, by not generating unnecessary branches when it can
prove which way a branch will go::

    >>> from peak.util.assembler import NotAConstant

    >>> def And(values, code=None):
    ...     if code is None:
    ...         return ast_curry(And, tuple(values))
    ...     end = Label()
    ...     for value in values[:-1]:
    ...         try:
    ...             if const_value(value):
    ...                 continue    # true constants can be skipped
    ...             else:           # and false ones end the chain right away
    ...                 return code(value, end)
    ...         except NotAConstant:    # but non-constants require code
    ...             code(value, end.JUMP_IF_FALSE, Code.POP_TOP)
    ...     code(values[-1], end)

    >>> c = Code()
    >>> c.return_( And([1, 2]) )
    >>> dis(c.code())
      0           0 LOAD_CONST               1 (2)
                  3 RETURN_VALUE

    >>> c = Code()
    >>> c.return_( And([1, 2, Local('x')]) )
    >>> dis(c.code())
      0           0 LOAD_FAST                0 (x)
                  3 RETURN_VALUE

    >>> c = Code()
    >>> c.return_( And([Local('x'), False, 27]) )
    >>> dis(c.code())
      0           0 LOAD_FAST                0 (x)
                  3 JUMP_IF_FALSE            4 (to 10)
                  6 POP_TOP
                  7 LOAD_CONST               1 (False)
            >>   10 RETURN_VALUE


Setting the Code's Calling Signature
====================================

The simplest way to set up the calling signature for a ``Code`` instance is
to clone an existing function or code object's signature, using the
``Code.from_function()`` or ``Code.from_code()`` classmethods.  These methods
create a new ``Code`` instance whose calling signature (number and names of
arguments) matches that of the original function or code objects::

    >>> def f1(a,b,*c,**d):
    ...     pass

    >>> c = Code.from_function(f1)
    >>> f2 = new.function(c.code(), globals())

    >>> import inspect

    >>> inspect.getargspec(f1)
    (['a', 'b'], 'c', 'd', None)

    >>> inspect.getargspec(f2)
    (['a', 'b'], 'c', 'd', None)

Note that these constructors do not copy any actual *code* from the code
or function objects.  They simply copy the signature, and, if you set the
``copy_lineno`` keyword argument to a true value, they will also set the
created code object's ``co_firstlineno`` to match that of the original code or
function object::

    >>> c1 = Code.from_function(f1, copy_lineno=True)
    >>> c1.co_firstlineno
    1

If you create a ``Code`` instance from a function that has nested positional
arguments, the returned code object will include a prologue to unpack the
arguments properly::

    >>> def f3(a, (b,c), (d,(e,f))):
    ...     pass

    >>> f4 = new.function(Code.from_function(f3).code(), globals())
    >>> dis(f4)
      0           0 LOAD_FAST                1 (.1)
                  3 UNPACK_SEQUENCE          2
                  6 STORE_FAST               3 (b)
                  9 STORE_FAST               4 (c)
                 12 LOAD_FAST                2 (.2)
                 15 UNPACK_SEQUENCE          2
                 18 STORE_FAST               5 (d)
                 21 UNPACK_SEQUENCE          2
                 24 STORE_FAST               6 (e)
                 27 STORE_FAST               7 (f)

This is roughly the same code that Python would generate to do the same
unpacking process, and is designed so that the ``inspect`` module will
recognize it as an argument unpacking prologue::

    >>> inspect.getargspec(f3)
    (['a', ['b', 'c'], ['d', ['e', 'f']]], None, None, None)

    >>> inspect.getargspec(f4)
    (['a', ['b', 'c'], ['d', ['e', 'f']]], None, None, None)


Code Attributes
===============

``Code`` instances have a variety of attributes corresponding to either the
attributes of the Python code objects they generate, or to the current state
of code generation.

For example, the ``co_argcount`` and ``co_varnames`` attributes
correspond to those used in creating the code for a Python function.  If you
want your code to be a function, you can set them as follows::

    >>> c = Code()
    >>> c.co_argcount = 3
    >>> c.co_varnames = ['a','b','c']

    >>> c.LOAD_CONST(42)
    >>> c.RETURN_VALUE()

    >>> f = new.function(c.code(), globals())
    >>> f(1,2,3)
    42

    >>> import inspect
    >>> inspect.getargspec(f)
    (['a', 'b', 'c'], None, None, None)

Although Python code objects want ``co_varnames`` to be a tuple, ``Code``
instances use a list, so that names can be added during code generation.  The
``.code()`` method automatically creates tuples where necessary.

Here are all of the ``Code`` attributes you may want to read or write:

co_filename
    A string representing the source filename for this code.  If it's an actual
    filename, then tracebacks that pass through the generated code will display
    lines from the file.  The default value is ``'<generated code>'``.

co_name
    The name of the function, class, or other block that this code represents.
    The default value is ``'<lambda>'``.

co_argcount
    Number of positional arguments a function accepts; defaults to 0

co_varnames
    A list of strings naming the code's local variables, beginning with its
    positional argument names, followed by its ``*`` and ``**`` argument names,
    if applicable, followed by any other local variable names.  These names
    are used by the ``LOAD_FAST`` and ``STORE_FAST`` opcodes, and invoking
    the ``.LOAD_FAST(name)`` and ``.STORE_FAST(name)`` methods of a code object
    will automatically add the given name to this list, if it's not already
    present.

co_flags
    The flags for the Python code object.  This defaults to
    ``CO_OPTIMIZED | CO_NEWLOCALS``, which is the correct value for a function
    using "fast" locals.  This value is automatically or-ed with ``CO_NOFREE``
    when generating a code object, if the ``co_cellvars`` and ``co_freevars``
    attributes are empty.  And if you use the ``LOAD_NAME()``,
    ``STORE_NAME()``, or ``DELETE_NAME()`` methods, the ``CO_OPTIMIZED`` bit
    is automatically reset, since these opcodes can only be used when the
    code is running with a real (i.e. not virtualized) ``locals()`` dictionary.

    If you need to change any other flag bits besides the above, you'll need to
    set or clear them manually.  For your convenience, the
    ``peak.util.assembler`` module exports all the ``CO_`` constants used by
    Python.  For example, you can use ``CO_VARARGS`` and ``CO_VARKEYWORDS`` to
    indicate whether a function accepts ``*`` or ``**`` arguments, as long as
    you extend the ``co_varnames`` list accordingly.  (Assuming you don't have
    an existing function or code object with the desired signature, in which
    case you could just use the ``from_function()`` or ``from_code()``
    classmethods instead of messing with these low-level attributes and flags.)

stack_size
    The predicted height of the runtime value stack, as of the current opcode.
    Its value is automatically updated by most opcodes, but you may want to
    save and restore it for things like try/finally blocks.  If you increase
    the value of this attribute, you should also update the ``co_stacksize``
    attribute if it is less than the new ``stack_size``.

co_freevars
    A tuple of strings naming a function's "cell" variables.  Defaults to an
    empty tuple.  A function's free variables are the variables it "inherits"
    from its surrounding scope.  If you're going to use this, you should set
    it only once, before generating any code that references any free *or* cell
    variables.

co_cellvars
    A tuple of strings naming a function's "cell" variables.  Defaults to an
    empty tuple.  A function's cell variables are the variables that are
    "inherited" by one or more of its nested functions.  If you're going to use
    this, you should set it only once, before generating any code that
    references any free *or* cell variables.

These other attributes are automatically generated and maintained, so you'll
probably never have a reason to change them:

co_consts
    A list of constants used by the code; the first (zeroth?) constant is
    always ``None``.  Normally, this is automatically maintained; the
    ``.LOAD_CONST(value)`` method checks to see if the constant is already
    present in this list, and adds it if it is not there.

co_names
    A list of non-optimized or global variable names.  It's automatically
    updated whenever you invoke a method to generate an opcode that uses
    such names.

co_code
    A byte array containing the generated code.  Don't mess with this.

co_firstlineno
    The first line number of the generated code.  It automatically gets set
    if you call ``.set_lineno()`` before generating any code; otherwise it
    defaults to zero.

co_lnotab
    A byte array containing a generated line number table.  It's automatically
    generated, so don't mess with it.

co_stacksize
    The maximum amount of stack space the code will require to run.  This
    value is usually updated automatically as you generate code.  However, if
    you manually set a new ``stack_size`` that is larger than the current
    ``co_stacksize``, you should increase the ``co_stacksize`` to match, so
    that ``co_stacksize`` is always the largest stack size the code will
    generate at runtime.


Blocks, Loops, and Exception Handling
=====================================

The Python ``SETUP_FINALLY``, ``SETUP_EXCEPT``, and ``SETUP_LOOP`` opcodes
all create "blocks" that go on the frame's "block stack" at runtime.  Each of
these opcodes *must* be matched with *exactly one* ``POP_BLOCK`` opcode -- no
more, and no less.  ``Code`` objects enforce this using an internal block stack
that matches each setup with its corresponding ``POP_BLOCK``.  Trying to pop
a nonexistent block, or trying to generate code when unclosed blocks exist is
an error::

    >>> c = Code()
    >>> c.POP_BLOCK()
    Traceback (most recent call last):
      ...
    AssertionError: Not currently in a block

    >>> c.SETUP_FINALLY()
    >>> c.code()
    Traceback (most recent call last):
      ...
    AssertionError: 1 unclosed block(s)

    >>> c.POP_BLOCK()
    >>> c.code()
    <code object <lambda> ...>

``Code`` objects also check that the stack level as of a ``POP_BLOCK`` is the
same as it was when the block was set up::

    >>> c = Code()
    >>> c.SETUP_LOOP()
    >>> c.LOAD_CONST(23)
    >>> c.POP_BLOCK()
    Traceback (most recent call last):
      ...
    AssertionError: Stack level mismatch: actual=1 expected=0


Exception Stack Size Adjustment
-------------------------------

When you ``POP_BLOCK`` for a ``SETUP_EXCEPT`` or ``SETUP_FINALLY``, the code's
maximum stack size is raised to ensure that it's at least 3 items higher than
the current stack size.  That way, there will be room for the items that Python
puts on the stack when jumping to a block's exception handling code::

    >>> c = Code()
    >>> c.SETUP_FINALLY()
    >>> c.stack_size, c.co_stacksize
    (0, 0)
    >>> c.POP_BLOCK()
    >>> c.END_FINALLY()
    >>> c.stack_size, c.co_stacksize
    (0, 3)

As you can see, the current stack size is unchanged, but the maximum stack size
has increased.  This increase is relative to the current stack size, though;
it's not an absolute increase::

    >>> c = Code()
    >>> c(1,2,3,4, *[Code.POP_TOP]*4)   # push 4 things, then pop 'em
    >>> c.SETUP_FINALLY()
    >>> c.POP_BLOCK()
    >>> c.END_FINALLY()
    >>> c.stack_size, c.co_stacksize
    (0, 4)

And this stack adjustment doesn't happen for loops, because they don't have
exception handlers::

    >>> c = Code()
    >>> c.SETUP_LOOP()
    >>> break_to = c.POP_BLOCK()
    >>> c.stack_size, c.co_stacksize
    (0, 0)


Try/Except Blocks
-----------------

In the case of ``SETUP_EXCEPT``, the *current* stack size is also increased by
3, because the code following the ``POP_BLOCK`` will be the exception handler
and will thus always have exception items on the stack::

    >>> c = Code()
    >>> c.SETUP_EXCEPT()
    >>> else_ = c.POP_BLOCK()
    >>> c.stack_size, c.co_stacksize
    (3, 3)

When a ``POP_BLOCK()`` is matched with a ``SETUP_EXCEPT``, it automatically
emits a ``JUMP_FORWARD`` and returns a forward reference that should be called
back when the "else" clause or end of the entire try/except statement is
reached::

    >>> c.POP_TOP()     # get rid of exception info
    >>> c.POP_TOP()
    >>> c.POP_TOP()
    >>> else_()
    >>> c.return_()
    >>> dis(c.code())
      0           0 SETUP_EXCEPT             4 (to 7)
                  3 POP_BLOCK
                  4 JUMP_FORWARD             3 (to 10)
            >>    7 POP_TOP
                  8 POP_TOP
                  9 POP_TOP
            >>   10 LOAD_CONST               0 (None)
                 13 RETURN_VALUE

In the example above, an empty block executes with an exception handler that
begins at offset 7.  When the block is done, it jumps forward to the end of
the try/except construct at offset 10.  The exception handler does nothing but
remove the exception information from the stack before it falls through to the
end.

Note, by the way, that it's usually easier to use labels to define blocks
like this::

    >>> c = Code()
    >>> done = Label()
    >>> c(
    ...     done.SETUP_EXCEPT,
    ...     done.POP_BLOCK,
    ...         Code.POP_TOP, Code.POP_TOP, Code.POP_TOP,
    ...     done,
    ...     Return()
    ... )

    >>> dis(c.code())
      0           0 SETUP_EXCEPT             4 (to 7)
                  3 POP_BLOCK
                  4 JUMP_FORWARD             3 (to 10)
            >>    7 POP_TOP
                  8 POP_TOP
                  9 POP_TOP
            >>   10 LOAD_CONST               0 (None)
                 13 RETURN_VALUE

Labels have a ``POP_BLOCK`` attribute that you can pass in when generating
code.


Try/Finally Blocks
------------------

When a ``POP_BLOCK()`` is matched with a ``SETUP_FINALLY``, it automatically
emits a ``LOAD_CONST(None)``, so that when the corresponding ``END_FINALLY``
is reached, it will know that the "try" block exited normally.  Thus, the
normal pattern for producing a try/finally construct is as follows::

    >>> c = Code()
    >>> c.SETUP_FINALLY()
    >>> # "try" suite goes here
    >>> c.POP_BLOCK()
    >>> # "finally" suite goes here
    >>> c.END_FINALLY()

And it produces code that looks like this::

    >>> dis(c.code())
      0           0 SETUP_FINALLY            4 (to 7)
                  3 POP_BLOCK
                  4 LOAD_CONST               0 (None)
            >>    7 END_FINALLY

The ``END_FINALLY`` opcode will remove 1, 2, or 3 values from the stack at
runtime, depending on how the "try" block was exited.  In the case of simply
"falling off the end" of the "try" block, however, the inserted
``LOAD_CONST(None)`` puts one value on the stack, and that one value is popped
off by the ``END_FINALLY``.  For that reason, ``Code`` objects treat
``END_FINALLY`` as if it always popped exactly one value from the stack, even
though at runtime this may vary.  This means that the estimated stack levels
within the "finally" clause may not be accurate -- which is why ``POP_BLOCK()``
adjusts the maximum expected stack size to accomodate up to three values being
put on the stack by the Python interpreter for exception handling.


Loops
-----

The ``POP_BLOCK`` for a loop marks the end of the loop body, and the beginning
of the "else" clause, if there is one.  It returns a forward reference that
should be called back either at the end of the "else" clause, or immediately if
there is no "else".  Any ``BREAK_LOOP`` opcodes that appear in the loop body
will jump ahead to the point at which the forward reference is resolved.

Here, we'll generate a loop that counts down from 5 to 0, with an "else" clause
that returns 42.  Three labels are needed: one to mark the end of the overall
block, one that's looped back to, and one that marks the "else" clause::

    >>> c = Code()
    >>> block = Label()
    >>> loop = Label()
    >>> else_ = Label()
    >>> c(
    ...     block.SETUP_LOOP,
    ...         5,      # initial setup - this could be a GET_ITER instead
    ...     loop,
    ...         else_.JUMP_IF_FALSE,        # while x:
    ...         1, Code.BINARY_SUBTRACT,    #     x -= 1
    ...         loop.CONTINUE_LOOP,
    ...     else_,                          # else:
    ...         Code.POP_TOP,
    ...     block.POP_BLOCK,
    ...         Return(42),                 #     return 42
    ...     block,
    ...     Return()
    ... )

    >>> dis(c.code())
      0           0 SETUP_LOOP              19 (to 22)
                  3 LOAD_CONST               1 (5)
            >>    6 JUMP_IF_FALSE            7 (to 16)
                  9 LOAD_CONST               2 (1)
                 12 BINARY_SUBTRACT
                 13 JUMP_ABSOLUTE            6
            >>   16 POP_TOP
                 17 POP_BLOCK
                 18 LOAD_CONST               3 (42)
                 21 RETURN_VALUE
            >>   22 LOAD_CONST               0 (None)
                 25 RETURN_VALUE

    >>> eval(c.code())
    42


Break and Continue
------------------

The ``BREAK_LOOP`` and ``CONTINUE_LOOP`` opcodes can only be used inside of
an active loop::

    >>> c = Code()
    >>> c.BREAK_LOOP()
    Traceback (most recent call last):
      ...
    AssertionError: Not inside a loop

    >>> c.CONTINUE_LOOP(c.here())
    Traceback (most recent call last):
      ...
    AssertionError: Not inside a loop

And ``CONTINUE_LOOP`` is automatically replaced with a ``JUMP_ABSOLUTE`` if
it occurs directly inside a loop block::

    >>> c.SETUP_LOOP()
    >>> c.CONTINUE_LOOP(c.here())
    >>> c.BREAK_LOOP()
    >>> c.POP_BLOCK()()
    >>> dis(c.code())
      0           0 SETUP_LOOP               5 (to 8)
            >>    3 JUMP_ABSOLUTE            3
                  6 BREAK_LOOP
                  7 POP_BLOCK

In other words, ``CONTINUE_LOOP`` only really emits a ``CONTINUE_LOOP`` opcode
if it's inside some other kind of block within the loop, e.g. a "try" clause::

    >>> c = Code()
    >>> c.SETUP_LOOP()
    >>> loop = c.here()
    >>> c.SETUP_FINALLY()
    >>> c.CONTINUE_LOOP(loop)
    >>> c.POP_BLOCK()
    >>> c.END_FINALLY()
    >>> c.POP_BLOCK()()
    >>> dis(c.code())
      0           0 SETUP_LOOP              12 (to 15)
            >>    3 SETUP_FINALLY            7 (to 13)
                  6 CONTINUE_LOOP            3
                  9 POP_BLOCK
                 10 LOAD_CONST               0 (None)
            >>   13 END_FINALLY
                 14 POP_BLOCK




----------------------
Internals and Doctests
----------------------

Line number tracking::

    >>> def simple_code(flno, slno, consts=1, ):
    ...     c = Code()
    ...     c.set_lineno(flno)
    ...     for i in range(consts): c.LOAD_CONST(None)
    ...     c.set_lineno(slno)
    ...     c.RETURN_VALUE()
    ...     return c.code()

    >>> dis(simple_code(1,1))
      1           0 LOAD_CONST               0 (None)
                  3 RETURN_VALUE

    >>> simple_code(1,1).co_stacksize
    1

    >>> dis(simple_code(13,414))    # FAILURE EXPECTED IN PYTHON 2.3
     13           0 LOAD_CONST               0 (None)
    414           3 RETURN_VALUE

    >>> dis(simple_code(13,14,100))
     13           0 LOAD_CONST               0 (None)
                  3 LOAD_CONST               0 (None)
    ...
     14         300 RETURN_VALUE

    >>> simple_code(13,14,100).co_stacksize
    100

    >>> dis(simple_code(13,572,120))    # FAILURE EXPECTED IN Python 2.3
     13           0 LOAD_CONST               0 (None)
                  3 LOAD_CONST               0 (None)
    ...
    572         360 RETURN_VALUE


Stack size tracking::

    >>> c = Code()
    >>> c.LOAD_CONST(1)
    >>> c.POP_TOP()
    >>> c.LOAD_CONST(2)
    >>> c.LOAD_CONST(3)
    >>> c.co_stacksize
    2
    >>> c.BINARY_ADD()
    >>> c.LOAD_CONST(4)
    >>> c.co_stacksize
    2
    >>> c.LOAD_CONST(5)
    >>> c.LOAD_CONST(6)
    >>> c.co_stacksize
    4
    >>> c.POP_TOP()
    >>> c.stack_size
    3

Stack underflow detection/recovery, and global/local variable names::

    >>> c = Code()
    >>> c.LOAD_GLOBAL('foo')
    >>> c.stack_size
    1
    >>> c.STORE_ATTR('bar')     # drops stack by 2
    Traceback (most recent call last):
      ...
    AssertionError: Stack underflow

    >>> c.co_names  # 'bar' isn't added unless success
    ['foo']

    >>> c.LOAD_ATTR('bar')
    >>> c.co_names
    ['foo', 'bar']

    >>> c.DELETE_FAST('baz')
    >>> c.co_varnames
    ['baz']

    >>> dis(c.code())
      0           0 LOAD_GLOBAL              0 (foo)
                  3 LOAD_ATTR                1 (bar)
                  6 DELETE_FAST              0 (baz)

Sequence operators and stack tracking:



Function calls and raise::

    >>> c = Code()
    >>> c.LOAD_GLOBAL('locals')
    >>> c.CALL_FUNCTION()   # argc/kwargc default to 0
    >>> c.POP_TOP()
    >>> c.LOAD_GLOBAL('foo')
    >>> c.LOAD_CONST(1)
    >>> c.LOAD_CONST('x')
    >>> c.LOAD_CONST(2)
    >>> c.CALL_FUNCTION(1,1)    # argc, kwargc
    >>> c.POP_TOP()

    >>> dis(c.code())
      0           0 LOAD_GLOBAL              0 (locals)
                  3 CALL_FUNCTION            0
                  6 POP_TOP
                  7 LOAD_GLOBAL              1 (foo)
                 10 LOAD_CONST               1 (1)
                 13 LOAD_CONST               2 ('x')
                 16 LOAD_CONST               3 (2)
                 19 CALL_FUNCTION          257
                 22 POP_TOP

    >>> c = Code()
    >>> c.LOAD_GLOBAL('foo')
    >>> c.LOAD_CONST(1)
    >>> c.LOAD_CONST('x')
    >>> c.LOAD_CONST(2)
    >>> c.BUILD_MAP(0)
    >>> c.stack_size
    5
    >>> c.CALL_FUNCTION_KW(1,1)
    >>> c.POP_TOP()
    >>> c.stack_size
    0

    >>> c = Code()
    >>> c.LOAD_GLOBAL('foo')
    >>> c.LOAD_CONST(1)
    >>> c.LOAD_CONST('x')
    >>> c.LOAD_CONST(1)
    >>> c.BUILD_TUPLE(1)
    >>> c.CALL_FUNCTION_VAR(0,1)
    >>> c.POP_TOP()
    >>> c.stack_size
    0

    >>> c = Code()
    >>> c.LOAD_GLOBAL('foo')
    >>> c.LOAD_CONST(1)
    >>> c.LOAD_CONST('x')
    >>> c.LOAD_CONST(1)
    >>> c.BUILD_TUPLE(1)
    >>> c.BUILD_MAP(0)
    >>> c.CALL_FUNCTION_VAR_KW(0,1)
    >>> c.POP_TOP()
    >>> c.stack_size
    0

    >>> c = Code()
    >>> c.RAISE_VARARGS(0)
    >>> c.RAISE_VARARGS(1)
    Traceback (most recent call last):
      ...
    AssertionError: Stack underflow
    >>> c.LOAD_CONST(1)
    >>> c.RAISE_VARARGS(1)

    >>> dis(c.code())
      0           0 RAISE_VARARGS            0
                  3 LOAD_CONST               1 (1)
                  6 RAISE_VARARGS            1

Sequence building, unpacking, dup'ing::

    >>> c = Code()
    >>> c.LOAD_CONST(1)
    >>> c.LOAD_CONST(2)
    >>> c.BUILD_TUPLE(3)
    Traceback (most recent call last):
      ...
    AssertionError: Stack underflow

    >>> c.BUILD_LIST(3)
    Traceback (most recent call last):
      ...
    AssertionError: Stack underflow

    >>> c.BUILD_TUPLE(2)
    >>> c.stack_size
    1

    >>> c.UNPACK_SEQUENCE(2)
    >>> c.stack_size
    2
    >>> c.DUP_TOPX(3)
    Traceback (most recent call last):
      ...
    AssertionError: Stack underflow

    >>> c.DUP_TOPX(2)
    >>> c.stack_size
    4
    >>> c.LOAD_CONST(3)
    >>> c.BUILD_LIST(5)
    >>> c.stack_size
    1
    >>> c.UNPACK_SEQUENCE(5)
    >>> c.BUILD_SLICE(3)
    >>> c.stack_size
    3
    >>> c.BUILD_SLICE(3)
    >>> c.stack_size
    1
    >>> c.BUILD_SLICE(2)
    Traceback (most recent call last):
      ...
    AssertionError: Stack underflow

    >>> dis(c.code())
      0           0 LOAD_CONST               1 (1)
                  3 LOAD_CONST               2 (2)
                  6 BUILD_TUPLE              2
                  9 UNPACK_SEQUENCE          2
                 12 DUP_TOPX                 2
                 15 LOAD_CONST               3 (3)
                 18 BUILD_LIST               5
                 21 UNPACK_SEQUENCE          5
                 24 BUILD_SLICE              3
                 27 BUILD_SLICE              3

Stack levels for MAKE_FUNCTION/MAKE_CLOSURE::

    >>> c = Code()
    >>> c.MAKE_FUNCTION(0)
    Traceback (most recent call last):
      ...
    AssertionError: Stack underflow

    >>> c.LOAD_CONST(1)
    >>> c.LOAD_CONST(2) # simulate being a function
    >>> c.MAKE_FUNCTION(1)
    >>> c.stack_size
    1

    >>> c = Code()
    >>> c.MAKE_CLOSURE(0, 0)
    Traceback (most recent call last):
      ...
    AssertionError: Stack underflow

    >>> c.LOAD_CONST(1)
    >>> c.LOAD_CONST(2) # simulate being a function
    >>> c.MAKE_CLOSURE(1, 0)
    >>> c.stack_size
    1

    >>> c = Code()
    >>> c.LOAD_CONST(1)
    >>> c.LOAD_CONST(2)
    >>> c.LOAD_CONST(3) # simulate being a function
    >>> c.MAKE_CLOSURE(1, 1)
    >>> c.stack_size
    1


Labels and backpatching forward references::

    >>> c = Code()
    >>> where = c.here()
    >>> c.LOAD_CONST(1)
    >>> c.JUMP_IF_TRUE(where)
    Traceback (most recent call last):
      ...
    AssertionError: Relative jumps can't go backwards


"Call" combinations::


    >>> c = Code()
    >>> c.set_lineno(1)
    >>> c(Call(Global('foo'), [Local('q')],
    ...        [('x',Const(1))], Local('starargs'))
    ... )
    >>> c.RETURN_VALUE()
    >>> dis(c.code())
      1           0 LOAD_GLOBAL              0 (foo)
                  3 LOAD_FAST                0 (q)
                  6 LOAD_CONST               1 ('x')
                  9 LOAD_CONST               2 (1)
                 12 LOAD_FAST                1 (starargs)
                 15 CALL_FUNCTION_VAR      257
                 18 RETURN_VALUE


    >>> c = Code()
    >>> c.set_lineno(1)
    >>> c(Call(Global('foo'), [Local('q')], [('x',Const(1))],
    ...        None, Local('kwargs'))
    ... )
    >>> c.RETURN_VALUE()
    >>> dis(c.code())
      1           0 LOAD_GLOBAL              0 (foo)
                  3 LOAD_FAST                0 (q)
                  6 LOAD_CONST               1 ('x')
                  9 LOAD_CONST               2 (1)
                 12 LOAD_FAST                1 (kwargs)
                 15 CALL_FUNCTION_KW       257
                 18 RETURN_VALUE


Cloning::

    >>> c = Code.from_function(lambda (x,y):1, True)
    >>> dis(c.code())
      1           0 LOAD_FAST                0 (.0)
                  3 UNPACK_SEQUENCE          2
                  6 STORE_FAST               1 (x)
                  9 STORE_FAST               2 (y)

    >>> c = Code.from_function(lambda x,(y,(z,a,b)):1, True)
    >>> dis(c.code())
      1           0 LOAD_FAST                1 (.1)
                  3 UNPACK_SEQUENCE          2
                  6 STORE_FAST               2 (y)
                  9 UNPACK_SEQUENCE          3
                 12 STORE_FAST               3 (z)
                 15 STORE_FAST               4 (a)
                 18 STORE_FAST               5 (b)

Constant folding for *args and **kw::

    >>> c = Code()
    >>> c.return_(Call(Const(type), [], [], (1,)))
    >>> dis(c.code())
      0           0 LOAD_CONST               1 (<type 'int'>)
                  3 RETURN_VALUE


    >>> c = Code()
    >>> c.return_(Call(Const(dict), [], [], [], Const({'x':1})))
    >>> dis(c.code())
      0           0 LOAD_CONST               1 ({'x': 1})
                  3 RETURN_VALUE



Demo: "Computed Goto"/"Switch Statement"
========================================

Finally, to give an example of a creative way to abuse Python bytecode, here
is an implementation of a simple "switch/case/else" structure::

    >>> from peak.util.assembler import LOAD_CONST, POP_BLOCK

    >>> def Pass(code=None):
    ...     if code is None:
    ...         return Pass

    >>> def NewConst(value, code=None):
    ...     if code is None:
    ...         return ast_curry(NewConst, value)
    ...     code.emit_arg(LOAD_CONST, len(code.co_consts))
    ...     code.co_consts.append(value)
    ...     code.stackchange((0,1))

    >>> import sys
    >>> WHY_CONTINUE = {'2.3':5, '2.4':32, '2.5':32}[sys.version[:3]]

    >>> def Switch(expr, cases, default=Pass, code=None):
    ...     if code is None:
    ...         return ast_curry(Switch, expr, tuple(cases), default)
    ...
    ...     d = {}
    ...     else_block  = Label()
    ...     cleanup     = Label()
    ...     end_switch  = Label()
    ...
    ...     code(
    ...         end_switch.SETUP_LOOP,
    ...             Call(NewConst(d.get), [expr]),
    ...         else_block.JUMP_IF_FALSE,
    ...             WHY_CONTINUE, Code.END_FINALLY
    ...     )
    ...
    ...     cursize = code.stack_size
    ...     for key, value in cases:
    ...         d[const_value(key)] = code.here()
    ...         code(value, cleanup.JUMP_FORWARD)
    ...
    ...     code(
    ...         else_block,
    ...             Code.POP_TOP, default,
    ...         cleanup,
    ...             Code.POP_BLOCK,
    ...         end_switch
    ...     )

    >>> c = Code()
    >>> c.co_argcount=1
    >>> c(Switch(Local('x'), [(1,Return(42)),(2,Return("foo"))], Return(27)))
    >>> c.return_()

    >>> f = new.function(c.code(), globals())
    >>> f(1)
    42
    >>> f(2)
    'foo'
    >>> f(3)
    27

    >>> dis(c.code())
      0           0 SETUP_LOOP              36 (to 39)
                  3 LOAD_CONST               1 (<...method get of dict...>)
                  6 LOAD_FAST                0 (x)
                  9 CALL_FUNCTION            1
                 12 JUMP_IF_FALSE           18 (to 33)
                 15 LOAD_CONST               2 (...)
                 18 END_FINALLY
                 19 LOAD_CONST               3 (42)
                 22 RETURN_VALUE
                 23 JUMP_FORWARD            12 (to 38)
                 26 LOAD_CONST               4 ('foo')
                 29 RETURN_VALUE
                 30 JUMP_FORWARD             5 (to 38)
            >>   33 POP_TOP
                 34 LOAD_CONST               5 (27)
                 37 RETURN_VALUE
            >>   38 POP_BLOCK
            >>   39 LOAD_CONST               0 (None)
                 42 RETURN_VALUE


TODO
====

* AST introspection
    * ast_type(node): called function, Const, or node.__class__
      * tuples are Const if their contents are; no other types are Const
    * ast_children(node): tuple of argument values for curried types, const value,
      or empty tuple.  If node is a tuple, the value must be flattened.
    * is_const(node): ast_type(node) is Const

* Inline builtins (getattr, operator.getitem, etc.) to opcodes
    * Getattr/Op/Unary("symbol", arg1 [, arg2]) node types -> Call() if folding
    * Call() translates functions back to Ops if inlining

* Pretty printing and short-naming of ASTs

* Test NAME vs. FAST operators flag checks/sets

* Test code flags generation/cloning


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