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Default Parameters
Default parameters allow you to specify default values for function arguments. If a caller doesn’t provide a value for an argument with a default, the default value is used.
def greet(name, greeting="Hello"):
print(f"{greeting}, {name}!")
greet("Alice") # Output: Hello, Alice!
greet("Bob", "Hi") # Output: Hi, Bob!
Important Considerations:
- Order Matters: Default parameters must come after non-default parameters in the function definition.
# Correct
def func(a, b=2):
pass
# Incorrect - will raise a SyntaxError
def func(a=1, b):
pass
- Mutable Default Arguments: Be cautious when using mutable objects (like lists or dictionaries) as default parameters. The default is created once when the function is defined, not each time it’s called. This can lead to unexpected behavior.
def add_to_list(item, my_list=1):
my_list.append(item)
return my_list
print(add_to_list(1)) # Output: 1
print(add_to_list(2)) # Output: 1, 2 <-- Unexpected! The list persists between calls.
To avoid this, use None as the default and create the list inside the function:
def add_to_list(item, my_list=None):
if my_list is None:
my_list = 1
my_list.append(item)
return my_list
print(add_to_list(1)) # Output: 1
print(add_to_list(2)) # Output: 2