How to Use Default Values for Functions in Python
The aim of this page📝is to list 2 best practices for using default values in Python functions
1 min readSep 14, 2022
- consider the following one-liner illustrating the syntax of default value assignment
def printMessage(message="Hello World"): print(message)>>> printMessage()
Hello World
1. DEFAULT VALUES COME LAST
- rule: in a function definition, parameters with default values must come after those without default values
- parameters specified in a function definition with
defare a comma-separated list - as you may know, function parameters they can be made optional by assigning a default values to the parameter as illustrated in the example above
- …and you get a syntax error if you don’t sign function call with all arguments that don’t have an assigned default value
2. DEFAULT VALUES SHOULD BE USED ONLY ON IMMUTABLE TYPES
- rule: do not use mutable default values; use only immutable default values
- the default value for an argument is only evaluated once, at the time the enclosing def statement is executed
- the enclosing def statement is typically executed upon `import`
intsfloatstringtupple
- lots of newcomers to Python are affected by this
- remember that
defitself is a statement executed at runtime (there is no hoisting) - i.e. this is dynamic and no matter how many times you call that the function object is created only once with
def - this can also have other negative consequences when using e.g.
listsas default values - the fix is to use immutable-only for default function values
