What are default values for function parameters and how can they be specified?
Default values for function parameters in Python allow us to assign a default value to a parameter if no argument is provided during the function call. This feature provides flexibility and allows us to define functions that can be called with different sets of arguments. In Python, default values for function parameters are specified in the function definition using the assignment operator (=).
To specify a default value for a function parameter, we simply assign a value to the parameter in the function header. For example, consider the following function definition:
python
def greet(name="Anonymous"):
print("Hello, " + name + "!")
In this example, the `greet` function has a parameter called `name` with a default value of "Anonymous". If no argument is provided when calling the function, it will use the default value and print "Hello, Anonymous!". However, if an argument is provided, it will use the provided value instead. For instance:
python
greet() # Output: Hello, Anonymous!
greet("John") # Output: Hello, John!
It's important to note that default values are evaluated only once when the function is defined, not every time the function is called. This means that if a mutable object (e.g., a list or dictionary) is used as a default value and modified within the function, the changes will persist across multiple function calls.
python
def add_item(item, lst=[]):
lst.append(item)
return lst
print(add_item(1)) # Output: [1]
print(add_item(2)) # Output: [1, 2]
In the example above, the `add_item` function has a parameter called `lst` with a default value of an empty list. When the function is called without providing a value for `lst`, it uses the default list. However, if we call the function multiple times, the default list is modified and retains its state across calls.
To avoid this unexpected behavior, it is recommended to use immutable objects (e.g., None, integers, strings) as default values, and create a new mutable object within the function if needed.
python
def add_item(item, lst=None):
if lst is None:
lst = []
lst.append(item)
return lst
print(add_item(1)) # Output: [1]
print(add_item(2)) # Output: [2]
In this modified version of the `add_item` function, we use None as the default value for `lst` and create a new empty list within the function if `lst` is None. This ensures that a new list is used for each function call, preventing unexpected behavior.
Default values for function parameters in Python allow us to specify a default value that is used if no argument is provided during the function call. They are assigned in the function definition using the assignment operator (=). Default values are evaluated only once when the function is defined and can lead to unexpected behavior when mutable objects are used. It is recommended to use immutable objects as default values and create new mutable objects within the function if needed.
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