Patching

before_after is sugar over the Mock library. It’s recommended that you read the docs before using before_after, especially Where to patch.

Example usage

Practical example of testing race conditions

def before_fn(*a, **k):
    print 'before_fn called with', a, k

def after_fn(*a, **k):
    print 'after_fn called with', a, k

def hello(name, greeting='Hello'):
    print '{greeting} {name}!'.format(
        greeting=greeting, name=name)

with before_after.before('__main__.hello', before_fn):
    hello('World')

# before_fn called with ('World',) {}
# Hello World!

with before_after.after('__main__.hello', after_fn):
    hello('everybody', greeting='Hi there')

# Hi there everybody!
# after_fn called with ('everybody',) {'greeting': 'Hi there'}

Use with recursive functions

By default, before_after only calls the before_fn/after_fn function once. This is useful if you’re calling the original function within the before_fn/after_fn, since otherwise you’ll blow the stack. This behaviour can be disabled by passing once=False.

my_list = []

def before_fn(*a, **k):
    print 'calling my_append in before_fn'
    my_append(1)

def my_append(item):
    print 'appending', item, 'to my_list'
    my_list.append(item)
    print 'my_list is now', my_list

with before_after.before('__main__.my_append', before_fn):
    my_append(2)

# calling my_append in before_fn
# appending 1 to my_list
# my_list is now [1]
# appending 2 to my_list
# my_list is now [1, 2]

with before_after.before('__main__.my_append', before_fn, once=False):
    my_append(2)

# calling my_append in before_fn
# calling my_append in before_fn
# calling my_append in before_fn
# ...
# RuntimeError: maximum recursion depth exceeded while calling a Python object

It’s recommended that if you’re passing once=False that you make sure your program will exit cleanly!

Use with methods

Since v1.0.0 before_after can be used on function methods. Make sure your before_fn/after_fn accepts a self argument.

class Greeter(object):
    def __init__(self):
        self.greeted = []

    def greet(self, name):
        print 'Hi there', name
        self.greeted.append(name)
        print 'This is now a party of', len(self.greeted)

def after_fn(self, name):
    self.greet("{name}'s guest".format(name=name))

greeter = Greeter()

with before_after.after('__main__.Greeter.greet', after_fn):
    greeter.greet('Alice')

# Hi there Alice
# This is now a party of 1
# Hi there Alice's guest
# This is now a party of 2