A default parameter value supplies an argument when a function call omits that parameter. It makes common cases shorter while preserving the option to provide a different value. section 9.2.1
Use default values with these rules:
- Declare the value in the function definition, such as `def power(x, y=2):`. section 9.2.1
- Place parameters with defaults after parameters without defaults in the parameter list. section 9.2.1
- Omit the argument to accept the default, or pass an argument positionally or by parameter name to override it. section 9.2.1 section 9.2.2
- Combine defaults with keyword arguments when a function has many optional settings; callers can select named options without depending on their order. section 9.2.2
A power function
def power(x, y=2): r = 1 while y > 0: r = r * x y = y - 1 return r power(3, 3) # 27 power(3) # 9 power(y=2, x=3) # 9 section 9.2.1 section 9.2.2
Optional file information
A function can default several Boolean options to false and enable selected behavior by name: `fileinfo = list_file_info(size=True, mod_date=True)`. This requests file size and modification date information without enabling creation-date information. section 9.2.2
Avoid mutating a mutable default such as `[]`. Python creates that default object when the function is first compiled and reuses it for the program’s lifetime, so later calls can inherit earlier mutations. section 9.3.1
odds = odd_numbers([1, 5, 7, 9, 10]) odds [1, 5, 7, 9, 1, 5, 7, 9]
Run the repeated call to see the same default list accumulate values across invocations. section 9.3.1
