嵌套函数是否存在等效的覆盖? [英] Does an equivalent of override exist for nested functions?

问题描述

如果我有这个功能，我该怎么用我自己的自定义版本替换内部功能?

If I have this function, what should I do to replace the inner function with my own custom version?

def foo():
    def bar():
        # I want to change this
        pass

    # here starts a long list of functions I want to keep unchanged
    def baz():
        pass

使用类可以很容易地覆盖方法.不过，我不知道如何使用嵌套函数来做到这一点.将 foo 更改为类(或其他任何东西)不是一种选择，因为它来自我无法修改的给定导入模块.

Using classes this would be easily done overriding the method. Though, I can't figure out how to do that with nested functions. Changing foo to be a class (or anything else) is not an option because it comes from a given imported module I can't modify.

推荐答案

这里有一种方法，通过破解函数内部来创建一个做正确的事"的新 foo.(正如@DSM 所提到的).不幸的是，我们不能直接跳到 foo 函数并弄乱它的内部结构，因为它们大多被标记为只读，所以我们要做的是修改我们手动构建的副本.

Here's one way of doing it, creating a new foo that "does the right thing" by hacking the function internals. ( As mentioned by @DSM ). Unfortunately we cant just jump into the foo function and mess with its internals, as they're mostly marked read only, so what we have to do is modify a copy we construct by hand.

# Here's the original function
def foo():
  def bar():
    print("    In bar orig")
  def baz():
    print("  Calling bar from baz")
    bar()
  print("Foo calling bar:")
  bar()
  print("Foo calling baz:")
  baz()

# Here's using it
foo()

# Now lets override the bar function

import types

# This is our replacement function
def my_bar():
  print("   Woo hoo I'm the bar override")

# This creates a new code object used by our new foo function 
# based on the old foo functions code object.
foocode = types.CodeType(
    foo.func_code.co_argcount,
    foo.func_code.co_nlocals,
    foo.func_code.co_stacksize,
    foo.func_code.co_flags,
    foo.func_code.co_code,
    # This tuple is a new version of foo.func_code.co_consts
    # NOTE: Don't get this wrong or you will crash python.
    ( 
       foo.func_code.co_consts[0],
       my_bar.func_code,
       foo.func_code.co_consts[2],
       foo.func_code.co_consts[3],
       foo.func_code.co_consts[4]
    ),
    foo.func_code.co_names,
    foo.func_code.co_varnames,
    foo.func_code.co_filename,
    foo.func_code.co_name,
    foo.func_code.co_firstlineno,
    foo.func_code.co_lnotab,
    foo.func_code.co_freevars,
    foo.func_code.co_cellvars )

# This is the new function we're replacing foo with
# using our new code.
foo = types.FunctionType( foocode , {})

# Now use it
foo()

我很确定它不会捕获所有案例.但它适用于示例(对我来说是旧的 python 2.5.1 )

I'm pretty sure its not going to catch all cases. But it works for the example (for me on an old python 2.5.1 )

可以进行一些整理的丑陋部分是:

Ugly bits that could do with some tidy up are:

1. 传递给 CodeType 的巨大参数列表
2. 由 co_consts 构造的丑陋元组，仅覆盖一个成员.所有信息都在 co_consts 中以确定要替换哪个 - 因此更智能的函数可以做到这一点.我使用 print( foo.func_code.co_consts ) 手工挖掘内部结构.

1. The huge argument list being passed to CodeType
2. The ugly tuple constructed from co_consts overriding only one member. All the info is in co_consts to determine which to replace - so a smarter function could do this. I dug into the internals by hand using print( foo.func_code.co_consts ).

你可以通过解释器找到一些关于CodeType和FunctionType的信息命令 help( types.CodeType ).

You can find some information about the CodeType and FunctionType by using the interpreter command help( types.CodeType ).

更新:我觉得这太丑了，所以我构建了一个辅助函数来使它更漂亮.使用助手，您可以编写:

UPDATE: I thought this was too ugly so I built a helper function to make it prettier. With the helper you can write:

# Use our function to get a new version of foo with "bar" replaced by mybar    
foo = monkey_patch_fn( foo, "bar", my_bar )

# Check it works
foo()

这是monkey_patch_fn的实现:

# Returns a copy of original_fn with its internal function
# called name replaced with new_fn.
def monkey_patch_fn( original_fn, name, new_fn ):

  #Little helper function to pick out the correct constant
  def fix_consts(x):
    if x==None: return None
    try:
      if x.co_name == name:
        return new_fn.func_code
    except AttributeError, e:
        pass
    return x

  original_code = original_fn.func_code
  new_consts = tuple( map( fix_consts, original_code.co_consts ) )
  code_type_args = [
     "co_argcount", "co_nlocals", "co_stacksize", "co_flags", "co_code",
     "co_consts", "co_names", "co_varnames", "co_filename", "co_name",
     "co_firstlineno", "co_lnotab", "co_freevars", "co_cellvars" ]

  new_code = types.CodeType(
     *[ ( getattr(original_code,x) if x!="co_consts" else new_consts )
        for x in code_type_args ] )
  return types.FunctionType( new_code, {} )

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