Patching Python code at runtime

README.md

A rather dirty way to patch module code at runtime.

a.py

import b

class A(object):
    def call_to_b(self, s):
        myb = b.B()
        myb.fn(s)

b.py

class B(object):
    def fn(self, s):
        """This function needs to be patched.
        """
        print s

patch.py

import inspect
import textwrap

# `A` uses `B` and we want `B` to be altered

import b

b_B_fn_source = inspect.getsource(b.B.fn)
# Alternatively, can use getsourcelines to get an array of lines

b_B_fn_source = textwrap.dedent(b_B_fn_source)

# Make sure the source looks as we expect it to
assert('print s' in b_B_fn_source)

# Patch the code
b_B_fn_source = b_B_fn_source.replace('def fn(', 'def patched_fn(')
b_B_fn_source = b_B_fn_source.replace('print s', 'print "patched", s')

# This creates the function `patched_fn`
exec(b_B_fn_source)

# Replace the original function
b.B.fn = patched_fn

# Now module `a` will use the patched code
import a
my_a = a.A()
my_a.call_to_b('ohhi')

# expect: >>> patched ohhi

Note that this won't work with magic methods (methods that start with __)

Thanks for this!