What is monkey patching?

No, it's not like any of those things. It's simply the dynamic replacement of attributes at runtime.

For instance, consider a class that has a method get_data. This method does an external lookup (on a database or web API, for example), and various other methods in the class call it. However, in a unit test, you don't want to depend on the external data source - so you dynamically replace the get_data method with a stub that returns some fixed data.

Because Python classes are mutable, and methods are just attributes of the class, you can do this as much as you like - and, in fact, you can even replace classes and functions in a module in exactly the same way.

But, as a commenter pointed out, use caution when monkeypatching:

1. If anything else besides your test logic calls get_data as well, it will also call your monkey-patched replacement rather than the original -- which can be good or bad. Just beware.

2. If some variable or attribute exists that also points to the get_data function by the time you replace it, this alias will not change its meaning and will continue to point to the original get_data. (Why? Python just rebinds the name get_data in your class to some other function object; other name bindings are not impacted at all.)

A MonkeyPatch is a piece of Python code which extends or modifies other code at runtime (typically at startup).

A simple example looks like this:

from SomeOtherProduct.SomeModule import SomeClass

def speak(self):
    return "ook ook eee eee eee!"

SomeClass.speak = speak

Source: MonkeyPatch page on Zope wiki.

What is a monkey patch?

Simply put, monkey patching is making changes to a module or class while the program is running.

Example in usage

There's an example of monkey-patching in the Pandas documentation:

import pandas as pd
def just_foo_cols(self):
    """Get a list of column names containing the string 'foo'

    """
    return [x for x in self.columns if 'foo' in x]

pd.DataFrame.just_foo_cols = just_foo_cols # monkey-patch the DataFrame class
df = pd.DataFrame([list(range(4))], columns=["A","foo","foozball","bar"])
df.just_foo_cols()
del pd.DataFrame.just_foo_cols # you can also remove the new method

To break this down, first we import our module:

import pandas as pd

Next we create a method definition, which exists unbound and free outside the scope of any class definitions (since the distinction is fairly meaningless between a function and an unbound method, Python 3 does away with the unbound method):

def just_foo_cols(self):
    """Get a list of column names containing the string 'foo'

    """
    return [x for x in self.columns if 'foo' in x]

Next we simply attach that method to the class we want to use it on:

pd.DataFrame.just_foo_cols = just_foo_cols # monkey-patch the DataFrame class

And then we can use the method on an instance of the class, and delete the method when we're done:

df = pd.DataFrame([list(range(4))], columns=["A","foo","foozball","bar"])
df.just_foo_cols()
del pd.DataFrame.just_foo_cols # you can also remove the new method

Caveat for name-mangling

If you're using name-mangling (prefixing attributes with a double-underscore, which alters the name, and which I don't recommend) you'll have to name-mangle manually if you do this. Since I don't recommend name-mangling, I will not demonstrate it here.

Testing Example

How can we use this knowledge, for example, in testing?

Say we need to simulate a data retrieval call to an outside data source that results in an error, because we want to ensure correct behavior in such a case. We can monkey patch the data structure to ensure this behavior. (So using a similar method name as suggested by Daniel Roseman:)

import datasource

def get_data(self):
    '''monkey patch datasource.Structure with this to simulate error'''
    raise datasource.DataRetrievalError

datasource.Structure.get_data = get_data

And when we test it for behavior that relies on this method raising an error, if correctly implemented, we'll get that behavior in the test results.

Just doing the above will alter the Structure object for the life of the process, so you'll want to use setups and teardowns in your unittests to avoid doing that, e.g.:

def setUp(self):
    # retain a pointer to the actual real method:
    self.real_get_data = datasource.Structure.get_data
    # monkey patch it:
    datasource.Structure.get_data = get_data

def tearDown(self):
    # give the real method back to the Structure object:
    datasource.Structure.get_data = self.real_get_data

(While the above is fine, it would probably be a better idea to use the mock library to patch the code. mock's patch decorator would be less error prone than doing the above, which would require more lines of code and thus more opportunities to introduce errors. I have yet to review the code in mock but I imagine it uses monkey-patching in a similar way.)