Python closures

From DocForge

Closures are a great language feature that is provided by most major interpreted languages these days. Although the examples given here are specific to Python, once you understand closures in one language, you can probably handle them in any other.

The best way to describe what a closure is would be to give an example. Let's say you have a function like this:

def output(func):
    print func()

Now, this may seem pretty brainless, but many Python libraries require you to pass a function object as an argument, which is often used as a callback inside the function. We might use the previous function in the following manner:

def output(func):
    print func()
 
def hello():
    return "Hello, World!"
 
output(hello)

This, of course, would output the text "Hello, World!". I love programming tutorial memes.

But what if we wanted more flexibility in how our function worked? Say we had an array of phrases we wished to output; here's an extremely over-engineered approach we might take.

phrases = {
    'american':'Hello, World!',
    'english':'Pip pip!',
    'german':'Guten Tag!',
    'french':'Bonjour!'
}
 
def output(func):
    print func()
 
for key, phrase in phrases.items():
    def hello():
        return phrase
    output(hello)

Now that part was pretty straightforward, and besides reminding us that in python we can define a function just about anywhere (take *that*, Java!), we really didn't use closures at all. In this next example, we'll save our inner functions to a dictionary that we'll use later on.

phrases = {
    'american':'Hello, World!',
    'english':'Pip pip!',
    'german':'Guten Tag!',
    'french':'Bonjour!'
}
 
def output(func):
    print func()
 
my_dict = {}
 
for key, phrase in phrases.items():
    def hello():
        return phrase
    my_dict[key] = hello
 
for key in my_dict:
    output(my_dict[key])

Aha, but something doesn't seem to be working in this case! The issue is related to how Python deals with closures; they aren't "closed" until we hit a return statement.

There's an easy enough fix, though. We'll just wrap the whole for loop in its own function:

phrases = {
    'american':'Hello, World!',
    'english':'Pip pip!',
    'german':'Guten Tag!',
    'french':'Bonjour!'
}
 
def output(func):
    print func()
 
my_dict = {}
 
for item in phrases.items():
    def __loop_closure(key, phrase):
        def hello():
            return phrase
        my_dict[key] = hello
    __loop_closure(*item)
 
for key in my_dict:
    output(my_dict[key])