whats_on_the_telly(penguin=None):
       if penguin is None:
           penguin = []
       penguin.append("property of the zoo")
       return penguin

Function call semantics are described in more detail in section Calls.
A function call always assigns values to all parameters 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.

Changed in version 3.8: The "/" function parameter syntax may be used
to indicate positional-only parameters. See **PEP 570** for details.

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.

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.

**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.

See also:

  **PEP 3107** - Function Annotations
     The original specification for function annotations.

  **PEP 484** - Type Hints
     Definition of a standard meaning for annotations: type hints.

  **PEP 526** - Syntax for Variable Annotations
     Ability to type hint variable declarations, including class
     variables and instance variables

  **PEP 563** - Postponed Evaluation of Annotations
     Support for forward references within annotations by preserving
     annotations in a string form at runtime instead of eager
     evaluation.
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