def Define function
Last updated: 2026-09-22
def Define function
def defines a function with optional params and return.
| Category | Keywords & Statements |
|---|---|
| Kind | Keyword / Statement |
| Python Version | all |
📝 Syntax
def name(params):
body
[return value]▶ Example
Edit the code, press Run, and see the result in the output panel:
Output：
Hi, Alice!🏷️ Related Keywords / Statements
💡 Related Cases
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❓ FAQ
QIf a function body has no `return`, what does calling it return?
AIt returns `None`. `def` only defines a function object; the body runs only when called. A function with no `return` or only a bare `return` always returns `None`.
QWhy does modifying a global variable inside a function not take effect?
ABecause assigning a name inside a function creates a local variable by default and does not touch the outer variable of the same name. To modify a module-level global, you must declare `global` inside the function first.
QWhy do I get «missing 1 required positional argument» when calling a function?
AThe required parameters defined must all be supplied at each call. If you do not want to pass them every time, give the parameter a default (`def f(x=1)`), or pass it explicitly as a keyword argument `f(x=2)`.
QWhat is the difference between `def` and `lambda`?
A`def` can contain multiple statements, docstrings, and type annotations — it is more general. `lambda` can only be a single expression, suitable as an argument to functions like `sorted`, `map`, or `filter`. In most cases, prefer `def`.
