The Python Language Reference, Release 3.9.5 101 8.6. Function definitions PEP 563 - Postponed Evaluation of Annotations Support for forward references within annotations by preserv- ing annotations in a string form at runtime instead of eager evaluation. PEP 526 - Syntax for Variable Annotations Ability to type hint variable declarations, including class variables and instance variables PEP 484 - Type Hints Definition of a standard meaning for annotations: type hints. PEP 3107 - Function Annotations The original specification for function annotations. See also: Programmer’s note: Functions are first-class objects. A “$_{def}$” statement executed inside a function definition defines a local function that can be returned or passed around. Free variables used in the nested function can access the local variables of the function containing the def. See section Naming and binding for details. It is also possible to create anonymous functions (functions not bound to a name), for immediate use in expressions. This uses lambda expressions, described in section Lambdas . Note that the lambda expression is merely a shorthand for a simplified function definition; a function defined in a “$_{def}$” statement can be passed around or assigned to another name just like a function defined by a lambda expression. The “$_{def}$” form is actually more powerful since it allows the execution of multiple statements and annotations. Parameters may have an annotation of the form “$_{: expression}$ ” following the parameter name. Any parameter may have an annotation, even those of the form$_{*identifier}$or$_{**identifier}$ . Functions may have “return” annotation of the form “$_{-> expression}$ ” after the parameter list. These annotations can be any valid Python expression. The presence of annotations does not change the semantics of a function. The annotation values are available as values of a dictionary keyed by the parameters’ names in the __annotations__ attribute of the function object. If the$_{annotations}$import from$_{__future__}$ is used, annotations are preserved as strings at runtime which enables postponed evaluation. Otherwise, they are evaluated when the function definition is executed. In this case annotations may be evaluated in a different order than they appear in the source code. Changed in version 3.8: The$_{/}$function parameter syntax may be used to indicate positional-only parameters. See PEP 570 for details. Function call semantics are described in more detail in section Calls . A function call always assigns values to all pa- rameters mentioned in the parameter list, either from positional arguments, from keyword arguments, or from default values. If the form “$_{*identifier}$” is present, it is initialized to a tuple receiving any excess positional parameters, defaulting to the empty tuple. If the form “$_{**identifier}$ ” is present, it is initialized to a new ordered mapping receiving any excess keyword arguments, defaulting to a new empty mapping of the same type. Parameters after “$_{*}$ ” or “$_{*identifier}$” are keyword-only parameters and may only be passed by keyword arguments. Parameters before “$_{/}$ ” are positional-only parameters and may only be passed by positional arguments. Default parameter values are evaluated from left to right when the function definition is executed. This means that the expression is evaluated once, when the function is defined, and that the same “pre-computed” value is used for each call. This is especially important to understand when a default parameter is a mutable object, such as a list or a dictionary: if the function modifies the object (e.g. by appending an item to a list), the default value is in effect modified. This is generally not what was intended. A way around this is to use$_{None}$ as the default, and explicitly test for it in the body of the function, e.g.: When one or more parameters have the form parameter$_{=}$expression , the function is said to have “default parameter values.” For a parameter with a default value, the corresponding argument may be omitted from a call, in which case the parameter’s default value is substituted. If a parameter has a default value, all following parameters up until the “$_{*}$ ” must also have a default value — this is a syntactic restriction that is not expressed by the grammar. Changed in version 3.9: Functions may be decorated with any valid$_{assignment_expression}$ . Previously, the grammar was much more restrictive; see PEP 614 for details. def whats_on_the_telly (penguin = None ): if penguin is None : penguin =[] penguin.append( "property of the zoo ") return penguin