Background
In the last couple of posts, we saw how we can pass multiple arguments in python functions and what are generators in python. Next, I want to explain what decorators are in python. Decorator is a very powerful feature of python but in order to explain it, we need to understand closure in Python. You must have heard of Javascript closures, these are kind of similar. We will see this in detail now.
Understanding python closure
Python closures are related to nested functions. Consider the following example:
def print_func(text):
text_to_print = text
def printer():
print(text_to_print)
return printer
print_function_reference = print_func("Hello World!")
print(print_function_reference)
print_function_reference()
What happens when you execute the above piece of code? It prints:
<function print_func.<locals>.printer at 0x7f21b16d1950>
Hello World!
<function print_func.<locals>.printer at 0x7f21b16d1950>
Hello World!
So what's happening here?
We have defined a function called print_func which takes in a string argument which we like to print. Then this method returns a reference new method called printer() and when you invoke this method(reference) you see your value is printed.
But wait a second? I am good with the part where I get a reference of printer method as seen in output but when I invoke it how does it get the value of text_to_print? It does not seem to be in printer methods scope.
>> This is exactly what we call closure.
A couple of other pointers before we go to the definition of closure:
- printer() function is called a nested function
- A nested function has read-only access of variables defined in the outer scope. 'text_to_print' in this case.
- Such variables are called "non-local" variables.
- Everything in python is an object with set of attributes. Yes, even a function. They all have some common attributes like "__doc__".
So, Closure is a function object that is used to access variables from enclosing scope, even though they are not present in the memory(out of scope). Eg. 'text_to_print' in this case.
They are used to invoke functions not in scope. Eg. printer() in above case. Scope for printer() function is inside print_func() function yet we can invoke it from outside.
They are used to invoke functions not in scope. Eg. printer() in above case. Scope for printer() function is inside print_func() function yet we can invoke it from outside.
NOTE: You can try deleting print_func() and then invoke print_function_reference(). It will still work, even though it's closing function is deleted.
When and Why to use Closures?
As you can see closure help with data hiding. You don't need to define global variables. That's exactly the primary use case of closures.
They are used in callbacks and decorators. We will have a detailed post on decorators (stay tuned!).
Typically when you have a few methods you can go with closure. However, if you have multiple methods you are better off with a class.
They are used in callbacks and decorators. We will have a detailed post on decorators (stay tuned!).
Typically when you have a few methods you can go with closure. However, if you have multiple methods you are better off with a class.
You can also see closure contents as follows:
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