def Executes at Runtime

The Python def is a true executable statement: when it runs, it creates a new function object and assigns it to a name. (Remember, all we have in Python is runtime; there is no such thing as a separate compile time.) Because it’s a statement, a def can appear anywhere a statement can—even nested in other statements. For instance, although defs normally are run when the module enclosing them is imported, it’s also completely legal to nest a function def inside an if statement to select between alternative definitions:

if test:
    def func():            # Define func this way
        ...
else:
    def func():            # Or else this way
        ...
...
func()                     # Call the version selected and built

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One way to understand this code is to realize that the def is much like an = statement: it simply assigns a name at runtime. Unlike in compiled languages such as C, Python functions do not need to be fully defined before the program runs. More generally, defs are not evaluated until they are reached and run, and the code inside defs is not evaluated until the functions are later called.

Because function definition happens at runtime, there’s nothing special about the function name. What’s important is the object to which it refers:

othername = func           # Assign function object
othername()                # Call func again

Here, the function was assigned to a different name and called through the new name. Like everything else in Python, functions are just objects; they are recorded explicitly in memory at program execution time. In fact, besides calls, functions allow arbitrary attributes to be attached to record information for later use:

def func(): ...            # Create function object
func()                     # Call object
func.attr = value          # Attach attributes