- 5.1 Function Definitions
- 5.2 Default Arguments
- 5.3 Variadic Arguments
- 5.4 Keyword Arguments
- 5.5 Variadic Keyword Arguments
- 5.6 Functions Accepting All Inputs
- 5.7 Positional-Only Arguments
- 5.8 Names, Documentation Strings, and Type Hints
- 5.9 Function Application and Parameter Passing
- 5.10 Return Values
- 5.11 Error Handling
- 5.12 Scoping Rules
- 5.13 Recursion
- 5.14 The lambda Expression
- 5.15 Higher-Order Functions
- 5.16 Argument Passing in Callback Functions
- 5.17 Returning Results from Callbacks
- 5.18 Decorators
- 5.19 Map, Filter, and Reduce
- 5.20 Function Introspection, Attributes, and Signatures
- 5.21 Environment Inspection
- 5.22 Dynamic Code Execution and Creation
- 5.23 Asynchronous Functions and await
- 5.24 Final Words: Thoughts on Functions and Composition
This chapter is from the book
This chapter is from the book
This chapter is from the book
5.2 Default Arguments
You can attach default values to function parameters by assigning values in the function definition. For example:
def split(line, delimiter=','):
statements
When a function defines a parameter with a default value, that parameter and all the parameters that follow it are optional. It is not possible to specify a parameter with no default value after any parameter with a default value.
Default parameter values are evaluated once when the function is first defined, not each time the function is called. This often leads to surprising behavior if mutable objects are used as a default:
def func(x, items=[]):
items.append(x)
return items
func(1) # returns [1]
func(2) # returns [1, 2]
func(3) # returns [1, 2, 3]
Notice how the default argument retains the modifications made from previous invocations. To prevent this, it is better to use None and add a check as follows:
def func(x, items=None):
if items is None:
items = []
items.append(x)
return items
As a general practice, to avoid such surprises, only use immutable objects for default argument values—numbers, strings, Booleans, None, and so on.