Question

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

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.

Answer 1

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()

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 )

Ugly bits that could do with some tidy up are:

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 ).

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

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()

Here’s the implementation of 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, {} )