python - 来自外部作用域的函数局部名称绑定(bind)

Question

我需要一种方法将名称从外部代码块“注入(inject)”到函数中，这样它们就可以在本地访问并且它们不需要由函数的代码专门处理(定义为函数参数，从*args等加载)

简化场景:提供一个框架，用户可以在其中定义(使用尽可能少的语法)自定义函数来操作框架的其他对象(不一定 全局).

理想情况下，用户定义

def user_func():
    Mouse.eat(Cheese)
    if Cat.find(Mouse):
        Cat.happy += 1

此处 Cat、Mouse 和 Cheese 是框架对象，出于充分的理由，它们不能绑定(bind)到全局命名空间。

我想为这个函数编写一个包装器，使其表现如下:

def framework_wrap(user_func):
    # this is a framework internal and has name bindings to Cat, Mouse and Cheese
    def f():
        inject(user_func, {'Cat': Cat, 'Mouse': Mouse, 'Cheese': Cheese})
        user_func()
    return f

然后这个包装器可以应用于所有用户定义的函数(作为装饰器，由用户自己或自动，尽管我打算使用元类)。

@framework_wrap
def user_func():

我知道 Python 3 的 nonlocal 关键字，但我仍然认为(从框架的用户角度)添加额外的一行很丑陋:

nonlocal Cat, Mouse, Cheese

并担心将他需要的每个对象添加到这一行。

非常感谢任何建议。

Answer 1

我越是把堆栈搞得一团糟，我就越希望我没有。不要破解全局变量来做你想做的事。而是破解字节码。我可以想到两种方法来做到这一点。

1) 添加包含您想要的引用的单元格到 f.func_closure 中。您必须重新组装函数的字节码以使用 LOAD_DEREF 而不是 LOAD_GLOBAL 并为每个值生成一个单元格。然后将单元格元组和新代码对象传递给 types.FunctionType 并获得具有适当绑定(bind)的函数。该函数的不同副本可以具有不同的本地绑定(bind)，因此它应该像您希望的那样是线程安全的。

2) 在函数参数列表的末尾为您的新局部变量添加参数。将适当的 LOAD_GLOBAL 替换为 LOAD_FAST。然后通过使用 types.FunctionType 并传入新代码对象和您希望作为默认选项的绑定(bind)元组来构造一个新函数。这是有限的，因为 python 将函数参数限制为 255，并且它不能用于使用可变参数的函数。尽管如此，我还是觉得这两者中更具挑战性，所以这就是我实现的那个(另外还有其他可以用这个完成的东西)。同样，您可以使用不同的绑定(bind)制作函数的不同副本，或者使用您希望从每个调用位置的绑定(bind)调用函数。因此它也可以像您希望的那样是线程安全的。

import types
import opcode

# Opcode constants used for comparison and replacecment
LOAD_FAST = opcode.opmap['LOAD_FAST']
LOAD_GLOBAL = opcode.opmap['LOAD_GLOBAL']
STORE_FAST = opcode.opmap['STORE_FAST']

DEBUGGING = True

def append_arguments(code_obj, new_locals):
    co_varnames = code_obj.co_varnames   # Old locals
    co_names = code_obj.co_names      # Old globals
    co_argcount = code_obj.co_argcount     # Argument count
    co_code = code_obj.co_code         # The actual bytecode as a string

    # Make one pass over the bytecode to identify names that should be
    # left in code_obj.co_names.
    not_removed = set(opcode.hasname) - set([LOAD_GLOBAL])
    saved_names = set()
    for inst in instructions(co_code):
        if inst[0] in not_removed:
            saved_names.add(co_names[inst[1]])

    # Build co_names for the new code object. This should consist of 
    # globals that were only accessed via LOAD_GLOBAL
    names = tuple(name for name in co_names
                  if name not in set(new_locals) - saved_names)

    # Build a dictionary that maps the indices of the entries in co_names
    # to their entry in the new co_names
    name_translations = dict((co_names.index(name), i)
                             for i, name in enumerate(names))

    # Build co_varnames for the new code object. This should consist of
    # the entirety of co_varnames with new_locals spliced in after the
    # arguments
    new_locals_len = len(new_locals)
    varnames = (co_varnames[:co_argcount] + new_locals +
                co_varnames[co_argcount:])

    # Build the dictionary that maps indices of entries in the old co_varnames
    # to their indices in the new co_varnames
    range1, range2 = xrange(co_argcount), xrange(co_argcount, len(co_varnames))
    varname_translations = dict((i, i) for i in range1)
    varname_translations.update((i, i + new_locals_len) for i in range2)

    # Build the dictionary that maps indices of deleted entries of co_names
    # to their indices in the new co_varnames
    names_to_varnames = dict((co_names.index(name), varnames.index(name))
                             for name in new_locals)

    if DEBUGGING:
        print "injecting: {0}".format(new_locals)
        print "names: {0} -> {1}".format(co_names, names)
        print "varnames: {0} -> {1}".format(co_varnames, varnames)
        print "names_to_varnames: {0}".format(names_to_varnames)
        print "varname_translations: {0}".format(varname_translations)
        print "name_translations: {0}".format(name_translations)


    # Now we modify the actual bytecode
    modified = []
    for inst in instructions(code_obj.co_code):
        # If the instruction is a LOAD_GLOBAL, we have to check to see if
        # it's one of the globals that we are replacing. Either way,
        # update its arg using the appropriate dict.
        if inst[0] == LOAD_GLOBAL:
            print "LOAD_GLOBAL: {0}".format(inst[1])
            if inst[1] in names_to_varnames:
                print "replacing with {0}: ".format(names_to_varnames[inst[1]])
                inst[0] = LOAD_FAST
                inst[1] = names_to_varnames[inst[1]]
            elif inst[1] in name_translations:    
                inst[1] = name_translations[inst[1]]
            else:
                raise ValueError("a name was lost in translation")
        # If it accesses co_varnames or co_names then update its argument.
        elif inst[0] in opcode.haslocal:
            inst[1] = varname_translations[inst[1]]
        elif inst[0] in opcode.hasname:
            inst[1] = name_translations[inst[1]]
        modified.extend(write_instruction(inst))

    code = ''.join(modified)
    # Done modifying codestring - make the code object

    return types.CodeType(co_argcount + new_locals_len,
                          code_obj.co_nlocals + new_locals_len,
                          code_obj.co_stacksize,
                          code_obj.co_flags,
                          code,
                          code_obj.co_consts,
                          names,
                          varnames,
                          code_obj.co_filename,
                          code_obj.co_name,
                          code_obj.co_firstlineno,
                          code_obj.co_lnotab)


def instructions(code):
    code = map(ord, code)
    i, L = 0, len(code)
    extended_arg = 0
    while i < L:
        op = code[i]
        i+= 1
        if op < opcode.HAVE_ARGUMENT:
            yield [op, None]
            continue
        oparg = code[i] + (code[i+1] << 8) + extended_arg
        extended_arg = 0
        i += 2
        if op == opcode.EXTENDED_ARG:
            extended_arg = oparg << 16
            continue
        yield [op, oparg]

def write_instruction(inst):
    op, oparg = inst
    if oparg is None:
        return [chr(op)]
    elif oparg <= 65536L:
        return [chr(op), chr(oparg & 255), chr((oparg >> 8) & 255)]
    elif oparg <= 4294967296L:
        return [chr(opcode.EXTENDED_ARG),
                chr((oparg >> 16) & 255),
                chr((oparg >> 24) & 255),
                chr(op),
                chr(oparg & 255),
                chr((oparg >> 8) & 255)]
    else:
        raise ValueError("Invalid oparg: {0} is too large".format(oparg))



if __name__=='__main__':
    import dis

    class Foo(object):
        y = 1

    z = 1
    def test(x):
        foo = Foo()
        foo.y = 1
        foo = x + y + z + foo.y
        print foo

    code_obj = append_arguments(test.func_code, ('y',))
    f = types.FunctionType(code_obj, test.func_globals, argdefs=(1,))
    if DEBUGGING:
        dis.dis(test)
        print '-'*20
        dis.dis(f)
    f(1)

请注意，此代码的整个分支(与 EXTENDED_ARG 相关的分支)未经测试，但对于常见情况，它似乎非常可靠。我将对其进行黑客攻击，目前正在编写一些代码来验证输出。然后(当我开始研究它时)我将针对整个标准库运行它并修复所有错误。

我也可能会实现第一个选项。