嵌套函数是否存在等价的覆盖？

Question

如果我有这个功能,我应该怎么做用我自己的自定义版本替换内部函数？

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

使用类可以轻松完成覆盖该方法.虽然,我无法弄清楚如何使用嵌套函数.更改foo为类(或其他任何内容)不是一个选项,因为它来自我无法修改的给定导入模块.

Answer 1

这是一种方法,通过破解函数内部创建一个"做正确的事"的新foo.(如@DSM所述).不幸的是,我们不能跳入foo函数并弄乱其内部,因为它们大部分都是标记为只读,所以我们要做的就是修改我们手工构建的副本.

# 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()

我很确定它不会抓住所有情况.但它适用于示例(对于我在旧的python 2.5.1上)

可以做一些整洁的丑陋的一点是:

1. 巨大的参数列表被传递给CodeType
2. 这个丑陋的元组co_consts只能覆盖一个成员.所有信息都在co_consts中以确定要替换的内容 - 因此更智能的功能可以做到这一点.我用手挖了内部print( foo.func_code.co_consts ).

你可以找到有关的一些信息CodeType,并FunctionType通过解释命令help( types.CodeType ).

更新:我认为这太丑了所以我构建了一个辅助函数,使其更漂亮.有了助手,你可以写:

# 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()

这是实施monkey_patch_fn:

# 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, {} )