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python_basics/functions/README.md
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python_basics/functions/README.md
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# 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(3,2)) # -> 4228
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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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## Arguments and Keywords
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