Do these 3 things before closing this tab:
1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteA Python function gives a task a name so you can call its code when needed, pass it inputs, and use any result it returns. Define one with def; the function is reusable within your program, while sharing it with another program requires separate module and import organization.
Define and call a function
Use def, a function name, parentheses, and a colon. The indented statements beneath it form the function body. Defining a function creates a function object and binds it to that name; the body runs when you call the function.
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def greet(name):
return f"Hello, {name}!"
message = greet("Sam")
print(message)Here, name is a parameter—the name in the definition—and "Sam" is an argument—the value supplied in the call. greet("Sam") returns a string, which the caller stores in message before displaying it. A function’s first statement may optionally be a string literal used as its documentation string, or docstring. See the Python Tutorial’s function definition guidance.
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Return a result when the caller needs it
return hands a value back to the caller. That value can be stored, combined with other values, or passed to another function. By contrast, print() displays something; it does not provide that displayed text as the function’s result. A function that reaches its end without a return expression returns None.
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def area(width, height):
return width * height
room_area = area(4, 3)
print(f"Room area: {room_area}")Use printing when displaying output is the function’s purpose. Return a value when other code should decide what to do with it. To provide multiple results, return a tuple, which the caller can unpack:
def dimensions():
return 4, 3
width, height = dimensions()For the distinction between returned results and output parameters, the Python Programming FAQ explains why returning values is the usual approach.
Understand what function scope means
A function call has its own local names. Parameters and names assigned inside the function are local by default, so assigning to a name inside a function does not ordinarily rebind a variable of the same name in the caller. Python resolves names through local, enclosing, global, and built-in scopes. global and nonlocal explicitly request rebinding in an outer scope; use them only when that behavior is intended.
Python’s argument model is often described as “passed by assignment”: the function receives a value that refers to an object. Rebinding a local parameter does not rebind the caller’s variable, but mutating a shared mutable object—such as a list—can be visible to the caller. This is why “passed by reference” on its own can be misleading. The Python FAQ discusses this distinction.
Choose arguments that make calls clear
Python supports positional and keyword arguments. Positional arguments are compact; keyword arguments identify which value is for which parameter. A function definition can also restrict parameters to positional-only or keyword-only use.
def describe(item, *, color):
return f"{color} {item}"
description = describe("mug", color="blue")The * makes color keyword-only, so its role is explicit in the call. Positional-only parameters, written before / in a definition, can be useful when an API should not expose a parameter name or when preserving compatibility matters. These are design choices, not performance guarantees; the Python Tutorial documents the syntax and conventions.
Use defaults carefully
A default makes an argument optional for callers. Python evaluates the default expression once, when it executes the function definition—not anew on every call. Immutable defaults such as a number or string are often straightforward. A mutable default, such as a list or dictionary, is the same object across calls, so changes can persist.
def add_item(item, items=None):
if items is None:
items = []
items.append(item)
return items
first = add_item("pen")
second = add_item("book")In this example, each call without an items argument creates its own list. If the caller supplies a list, the function appends to that supplied object. The behavior follows from when the default is evaluated; it is not a special exception raised by Python. The Python Tutorial describes default arguments and this mutable-default pitfall.
Use named functions for behavior you may reuse
Functions are objects: you can assign one to another name, pass it to another function, or return it from a function. A named def is usually easier to read when the logic needs explanation or documentation. A lambda can suit a small, single expression, but it does not replace a clear named function when behavior grows. The Python glossary and function tutorial cover functions as objects and lambda expressions.
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