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:
- The huge argument list being passed to CodeType
- 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, {} )