326 lines
8.2 KiB
Markdown
326 lines
8.2 KiB
Markdown
# Functions
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{:.no_toc}
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<nav markdown="1" class="toc-class">
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* TOC
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{:toc}
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</nav>
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## The goal
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Questions to [David Rotermund](mailto:davrot@uni-bremen.de)
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## [Most simple function](https://docs.python.org/3/tutorial/controlflow.html#defining-functions)
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These three functions are equivalent:
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```python
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def my_function():
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pass
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def my_function():
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return
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def my_function():
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return None
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```
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> return leaves the current function call with the expression list (or None) as return value.
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There is a return at the end, even if you don't put it there.
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## [def](https://docs.python.org/3/reference/compound_stmts.html?highlight=funcdef#function-definitions)
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```python
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funcdef ::= [decorators] "def" funcname [type_params] "(" [parameter_list] ")"
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["->" expression] ":" suite
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decorators ::= decorator+
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decorator ::= "@" assignment_expression NEWLINE
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parameter_list ::= defparameter ("," defparameter)* "," "/" ["," [parameter_list_no_posonly]]
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| parameter_list_no_posonly
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parameter_list_no_posonly ::= defparameter ("," defparameter)* ["," [parameter_list_starargs]]
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| parameter_list_starargs
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parameter_list_starargs ::= "*" [parameter] ("," defparameter)* ["," ["**" parameter [","]]]
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| "**" parameter [","]
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parameter ::= identifier [":" expression]
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defparameter ::= parameter ["=" expression]
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funcname ::= identifier
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```
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## [pass](https://docs.python.org/3/reference/simple_stmts.html#the-pass-statement)
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```python
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pass_stmt ::= "pass"
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```
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> pass is a null operation — when it is executed, nothing happens. It is useful as a placeholder when a statement is required syntactically, but no code needs to be executed
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>
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## [return](https://docs.python.org/3/reference/simple_stmts.html#the-return-statement)
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```python
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return_stmt ::= "return" [expression_list]
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```
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> return may only occur syntactically nested in a function definition, not within a nested class definition.
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>
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> If an expression list is present, it is evaluated, else None is substituted.
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>
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> return leaves the current function call with the expression list (or None) as return value.
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>
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> When return passes control out of a try statement with a finally clause, that finally clause is executed before really leaving the function.
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## [Return values](https://docs.python.org/3/reference/simple_stmts.html#return)
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### No return value
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As we stated above these functions all return None:
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```python
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def my_function():
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pass
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def my_function():
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return
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def my_function():
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return None
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```
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### One return value
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These functions return one value:
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```python
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def my_function():
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return 1
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def my_function():
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return "Hello"
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def my_function():
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return (1+1)/1
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```
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e.g.
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```python
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def my_function():
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return 1
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print(my_function())
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```
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### Many return values
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```python
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def my_function():
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return 2, "A", 79, 3.1314
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print(my_function()) # -> (2, 'A', 79, 3.1314)
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abcd = my_function() # -> (2, 'A', 79, 3.1314)
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print(abcd) # -> (2, 'A', 79, 3.1314)
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a, b, c, d = my_function()
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print(a) # -> 2
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print(b) # -> A
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print(c) # -> 79
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print(d) # -> 3.1314
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a, b, c = my_function() # ValueError: too many values to unpack (expected 3)
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```
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## [Functions are objects](https://docs.python.org/3/tutorial/controlflow.html#defining-functions)
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Functions are objects. Without **()** at the end, you interact with the function. With the **()** you use the function:
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```python
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def my_function():
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return 1
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print(my_function) # -> <function my_function at 0x7f956166a520>
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print(my_function()) # -> 1
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```
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Interesting, but why do I need to know this? Good question, imaginary person! We can use functions as "parameters" via arguments for other functions:
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```python
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def my_function_0():
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return 3
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def my_function_1():
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return 2
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def my_function_2(func):
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return func() ** 2
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print(my_function_2(my_function_0)) # -> 9
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print(my_function_2(my_function_1)) # -> 4
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```
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## [Default Argument Values](https://docs.python.org/3/tutorial/controlflow.html#default-argument-values)
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We can define a default value for arguments:
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```python
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def my_function(a=2, b=7, c=5, d=8):
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return a + b**2 + c**3 + d**4
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print(my_function()) # -> 4272
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```
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We can (partially) overwrite the default values:
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```python
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def my_function(a=2, b=7, c=5, d=8):
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return a + b**2 + c**3 + d**4
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print(my_function(b=2,a=3)) # -> 4228
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print(my_function(d=7)) # -> 2577
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print(my_function(3,2)) # -> 4228
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print(my_function(3,b=2)) # -> 4228
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print(my_function(b=2,3)) # SyntaxError: positional argument follows keyword argument
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```
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In the case of **my_function(3,2)**, the default values are replaced in the order the arguments of the function are defined. a,b,c, and finally d. When using the keyword e.g. d. Then I can overwrite only selected arguments.
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## [Important warning](https://docs.python.org/3/tutorial/controlflow.html#default-argument-values) -- Mutable objects
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> Important warning: **The default value is evaluated only once**. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes.
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Deadly:
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```python
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def f(a, L=[]):
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L.append(a)
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return L
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print(f(1)) # -> [1]
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print(f(2)) # -> [1, 2]
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print(f(3)) # -> [1, 2, 3]
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```
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Correct:
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```python
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def f(a, L=None):
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if L is None:
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L = []
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L.append(a)
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return L
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print(f(1)) # -> [1]
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print(f(2)) # -> [2]
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print(f(3)) # -> [3]
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```
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## Mutable arguments don't need return values
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```python
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def f(x):
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x.append(1)
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y = []
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f(y)
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print(y)
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```
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## [Arguments: Positional and Keywords](https://docs.python.org/3/tutorial/controlflow.html#special-parameters)
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> A function definition may look like:
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```python
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def f(pos1, pos2, /, pos_or_kwd, *, kwd1, kwd2):
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----------- ---------- ----------
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| | |
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| Positional or keyword |
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| - Keyword only
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-- Positional only
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```
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**This is the typical case:**
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[Positional-or-Keyword Arguments](https://docs.python.org/3/tutorial/controlflow.html#positional-or-keyword-arguments)
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> **If / and * are not present** in the function definition, arguments may be passed to a function by position or by keyword.
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[Keyword Arguments](https://docs.python.org/3/tutorial/controlflow.html#keyword-arguments)
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{: .topic-optional}
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This is an optional topic!
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> Functions can also be called using keyword arguments of the form kwarg=value.
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[Positional-Only Parameters](https://docs.python.org/3/tutorial/controlflow.html#positional-only-parameters)
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{: .topic-optional}
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This is an optional topic!
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> If positional-only, the parameters’ order matters, and the parameters cannot be passed by keyword.
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>
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> If there is no / in the function definition, there are no positional-only parameters.
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[Keyword-Only Arguments](https://docs.python.org/3/tutorial/controlflow.html#keyword-only-arguments)
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{: .topic-optional}
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This is an optional topic!
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> To mark parameters as keyword-only, indicating the parameters must be passed by keyword argument, place an * in the arguments list just before the first keyword-only parameter.
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## [Arbitrary Argument Lists](https://docs.python.org/3/tutorial/controlflow.html#arbitrary-argument-lists)
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```python
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def f(p1, *arguments, **keywords):
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print("p1:")
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print(p1)
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print()
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print("*arguments")
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for arg in arguments:
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print(arg)
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print()
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print("**keywords")
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for kw in keywords:
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print(f"{kw}: {keywords[kw]}")
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print()
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f(1, 2, 3, a=4, b=5)
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```
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Output:
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```python
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p1:
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1
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*arguments
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2
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3
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**keywords
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a: 4
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b: 5
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```
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[Unpacking Argument Lists](https://docs.python.org/3/tutorial/controlflow.html#unpacking-argument-lists)
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```python
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def f(a: int, b: int, c: int, d: int) -> int:
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return a * b * c * d
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print(f(2, 3, 4, 5)) # -> 120
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my_list_a = [2, 3, 4]
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my_list_b = [2, 3, 4, 5]
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my_list_c = [3, 4, 5]
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print(f(*my_list_a, 5)) # -> 120 (plus a false warning: 'Too many arguments for "f"' from MyPy)
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print(f(*my_list_b)) # -> 120
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print(f(2, *my_list_c)) # -> 120
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my_tuple_c = [3, 4, 5]
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print(f(2, *my_tuple_c)) # -> 120
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```
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## Documentation strings
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