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A Python function gives a useful operation a name so you can call it again with different inputs. Define it once with def, pass values as arguments, and use return when the caller needs a result back.
What a Python function does
A function is a named block of code. The Python Tutorial says, “The def keyword introduces a function definition.” Defining a function binds its name; its indented body runs when code calls that name. This lets you avoid copying the same operation into multiple places and gives that operation a clear purpose.
For example, a greeting can be defined once and reused for different people:
def make_greeting(name):
"""Return a greeting for one person."""
return f"Hello, {name}!"
first = make_greeting("Ari")
second = make_greeting("Sam")The function returns a string. It does not print anything by itself; the caller can store the returned string, combine it with other values, or print it. The docstring—the text in triple quotes—describes the function’s purpose and can be used by documentation tools.
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How to define and call a function
- Write
def, followed by a function name and parentheses. In the greeting example, the name ismake_greeting. - Put any input names inside the parentheses, then end the definition line with a colon. Here,
nameis the input name. - Indent the function body. The body contains the steps to perform; this example uses
returnto provide a result. - Call the function by writing its name with parentheses and supplying a value, as in
make_greeting("Ari").
Each call executes the body. Calling the function again with another value reuses the same behavior without duplicating its implementation.
Parameters and arguments: names versus values
A parameter is a name in the function definition; an argument is a value supplied when the function is called. In def make_greeting(name):, name is the parameter. In make_greeting("Ari"), "Ari" is the argument.
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Python lets callers supply arguments by position or by keyword, and a parameter can have a default value:
def make_greeting(name, greeting="Hello"):
return f"{greeting}, {name}!"
make_greeting("Ari")
make_greeting("Sam", greeting="Welcome")The first call uses the default greeting. The second names its argument explicitly. Positional arguments are compact but depend on order; keyword arguments make the supplied value clear. Defaults are useful when an input is genuinely optional. Python also supports positional-only and keyword-only parameter markers when an API needs to restrict how callers provide values; see the Python Tutorial’s function parameter documentation.
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Return a result or perform an action?
return sends a value back to the caller. That makes the result available for later computation, storage, or display. Printing is a side effect: it displays text, but does not hand that text back as the function’s result.
def print_greeting(name):
print(f"Hello, {name}!")
def get_greeting(name):
return f"Hello, {name}!"
result = print_greeting("Ari") # Displays the greeting; result is None
message = get_greeting("Ari") # message contains the greetingA function that reaches its end without returning an expression produces None. Writing return without an expression also returns None. Choose printing when the function’s job is to display something; return a value when other code needs to use the result. The Python Functional Programming HOWTO discusses this distinction in the context of functions and side effects.
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Why a default list can keep old values
Python evaluates a default argument expression once, when the function definition runs—not afresh for each call. A mutable default such as a list can therefore keep changes made by earlier calls:
def add_item(item, items=[]):
items.append(item)
return itemsCalls to this function share the same default list. If each call should start with a fresh list, use None as the default and create the list inside the function:
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def add_item(item, items=None):
if items is None:
items = []
items.append(item)
return itemsThis pattern also applies to dictionaries and other mutable objects. The Python Software Foundation’s Programming FAQ explains default-argument evaluation and the mutable-default pitfall.
When to make a function
Extract code into a function when a meaningful operation is repeated or when giving a sequence of steps a clear name makes the surrounding code easier to understand. Keep the function focused and choose a name that describes what it does. Not every short repeated line needs a function; the goal is a useful boundary that makes behavior easier to reuse or follow.
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