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Description:

patchy 2.8.0

Patch the inner source of python functions at runtime.
A quick example, making a function that returns 1 instead return 9001:
>>> def sample():
... return 1
...
>>> patchy.patch(
... sample,
... """\
... @@ -1,2 +1,2 @@
... def sample():
... - return 1
... + return 9001
... """,
... )
>>> sample()
9001
Patchy works by replacing the code attribute of the function, leaving the
function object itself the same. It’s thus more versatile than monkey patching,
since if the function has been imported in multiple places they’ll also call
the new behaviour.

Installation
Use pip:
python -m pip install patchy
Python 3.8 to 3.12 supported.

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Why?
If you’re monkey-patching an external library to add or fix some functionality,
you will probably forget to check the monkey patch when you upgrade it. By
using a patch against its source code, you can specify some context that you
expect to remain the same in the function that will be checked before the
source is applied.
I found this with some small but important patches to Django for a project.
Since it takes a lot of energy to maintain a fork, writing monkey patches was
the chosen quick solution, but then writing actual patches would be better.
The patches are applied with the standard patch commandline utility.

Why not?
There are of course a lot of reasons against:

It’s (relatively) slow (since it writes the source to disk and calls the
patch command)
If you have a patch file, why not just fork the library and apply it?
At least with monkey-patching you know what end up with, rather than having
the changes being done at runtime to source that may have changed.

All are valid arguments. However once in a while this might be the right
solution.

How?
The standard library function inspect.getsource() is used to retrieve the
source code of the function, the patch is applied with the commandline utility
patch, the code is recompiled, and the function’s code object is replaced
the new one. Because nothing tends to poke around at code objects apart from
dodgy hacks like this, you don’t need to worry about chasing any references
that may exist to the function, unlike mock.patch.
A little special treatment is given to instancemethod, classmethod, and
staticmethod objects to make sure the underlying function is what gets
patched and that you don’t have to worry about the details.

API

patch(func, patch_text)
Apply the patch patch_text to the source of function func. func may
be either a function, or a string providing the dotted path to import a
function.
If the patch is invalid, for example the context lines don’t match,
ValueError will be raised, with a message that includes all the output from
the patch utility.
Note that patch_text will be textwrap.dedent()’ed, but leading
whitespace will not be removed. Therefore the correct way to include the patch
is with a triple-quoted string with a backslash - """\ - which starts the
string and avoids including the first newline. A final newline is not required
and will be automatically added if not present.
Example:
import patchy

def sample():
return 1

patchy.patch(
sample,
"""\
@@ -2,2 +2,2 @@
- return 1
+ return 2""",
)

print(sample()) # prints 2

mc_patchface(func, patch_text)
An alias for patch, so you can meme it up by calling
patchy.mc_patchface().

unpatch(func, patch_text)
Unapply the patch patch_text from the source of function func. This is
the reverse of patch()ing it, and calls patch --reverse.
The same error and formatting rules apply as in patch().
Example:
import patchy

def sample():
return 2

patchy.unpatch(
sample,
"""\
@@ -2,2 +2,2 @@
- return 1
+ return 2""",
)

print(sample()) # prints 1

temp_patch(func, patch_text)
Takes the same arguments as patch. Usable as a context manager or function
decorator to wrap code with a call to patch before and unpatch after.
Context manager example:
def sample():
return 1234

patch_text = """\
@@ -1,2 +1,2 @@
def sample():
- return 1234
+ return 5678
"""

with patchy.temp_patch(sample, patch_text):
print(sample()) # prints 5678
Decorator example, using the same sample and patch_text:
@patchy.temp_patch(sample, patch_text)
def my_func():
return sample() == 5678

print(my_func()) # prints True

replace(func, expected_source, new_source)
Check that function or dotted path to function func has an AST matching
expected_source, then replace its inner code object with source compiled
from new_source. If the AST check fails, ValueError will be raised with
current/expected source code in the message. In the author’s opinion it’s
preferable to call patch() so your call makes it clear to see what is being
changed about func, but using replace() is simpler as you don’t have to
make a patch and there is no subprocess call to the patch utility.
Note both expected_source and new_source will be
textwrap.dedent()’ed, so the best way to include their source is with a
triple quoted string with a backslash escape on the first line, as per the
example below.
If you want, you can pass expected_source=None to avoid the guard against
your target changing, but this is highly unrecommended as it means if the
original function changes, the call to replace() will continue to silently
succeed.
Example:
import patchy

def sample():
return 1

patchy.replace(
sample,
"""\
def sample():
return 1
""",
"""\
def sample():
return 42
""",
)

print(sample()) # prints 42

How to Create a Patch

Save the source of the function of interest (and nothing else) in a .py
file, e.g. before.py:
def foo():
print("Change me")
Make sure you dedent it so there is no whitespace before the def, i.e.
d is the first character in the file. For example if you wanted to patch
the bar() method below:
class Foo:
def bar(self, x):
return x * 2
…you would put just the method in a file like so:
def bar(self, x):
return x * 2
However we’ll continue with the first example before.py since it’s
simpler.

Copy that .py file, to e.g. after.py, and make the changes you
want, such as:
def foo():
print("Changed")

Run diff, e.g. diff -u before.py after.py. You will get output like:
diff --git a/Users/chainz/tmp/before.py b/Users/chainz/tmp/after.py
index e6b32c6..31fe8d9 100644
--- a/Users/chainz/tmp/before.py
+++ b/Users/chainz/tmp/after.py
@@ -1,2 +1,2 @@
def foo():
- print("Change me")
+ print("Changed")

The filenames are not necessary for patchy to work. Take only from the
first @@ line onwards into the multiline string you pass to
patchy.patch():
patchy.patch(
foo,
"""\
@@ -1,2 +1,2 @@
def foo():
- print("Change me")
+ print("Changed")
""",
)