Create README.md

Signed-off-by: David Rotermund <54365609+davrot@users.noreply.github.com>

python_basics/functions/README.md

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+# Functions
+{:.no_toc}
+
+<nav markdown="1" class="toc-class">
+* TOC
+{:toc}
+</nav>
+
+## The goal
+
+
+
+Questions to [David Rotermund](mailto:davrot@uni-bremen.de)
+
+## [Most simple function](https://docs.python.org/3/tutorial/controlflow.html#defining-functions)
+
+These three functions are equivalent: 
+
+```python
+def my_function():
+    pass
+
+def my_function():
+    return
+
+def my_function():
+    return None
+```
+
+> return leaves the current function call with the expression list (or None) as return value.
+
+There is a return at the end, even if you don't put it there. 
+
+## [def](https://docs.python.org/3/reference/compound_stmts.html?highlight=funcdef#function-definitions) 
+
+```python
+funcdef                   ::=  [decorators] "def" funcname [type_params] "(" [parameter_list] ")"
+                               ["->" expression] ":" suite
+decorators                ::=  decorator+
+decorator                 ::=  "@" assignment_expression NEWLINE
+parameter_list            ::=  defparameter ("," defparameter)* "," "/" ["," [parameter_list_no_posonly]]
+                                 | parameter_list_no_posonly
+parameter_list_no_posonly ::=  defparameter ("," defparameter)* ["," [parameter_list_starargs]]
+                               | parameter_list_starargs
+parameter_list_starargs   ::=  "*" [parameter] ("," defparameter)* ["," ["**" parameter [","]]]
+                               | "**" parameter [","]
+parameter                 ::=  identifier [":" expression]
+defparameter              ::=  parameter ["=" expression]
+funcname                  ::=  identifier
+```
+
+## [pass](https://docs.python.org/3/reference/simple_stmts.html#the-pass-statement)
+
+```python
+pass_stmt ::=  "pass"
+```
+
+> 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
+> 
+
+## [return](https://docs.python.org/3/reference/simple_stmts.html#the-return-statement)
+
+```python
+return_stmt ::=  "return" [expression_list]
+```
+
+> return may only occur syntactically nested in a function definition, not within a nested class definition.
+>
+> If an expression list is present, it is evaluated, else None is substituted.
+>
+> return leaves the current function call with the expression list (or None) as return value.
+>
+> When return passes control out of a try statement with a finally clause, that finally clause is executed before really leaving the function.
+
+## [Return values​](https://docs.python.org/3/reference/simple_stmts.html#return)
+
+### No return value
+As we stated above these functions all return None:
+
+```python
+def my_function():
+    pass
+
+def my_function():
+    return
+
+def my_function():
+    return None
+```
+
+### One return value
+These functions return one value:
+
+```python
+def my_function():
+    return 1
+
+def my_function():
+    return "Hello"
+
+def my_function():
+    return (1+1)/1
+```
+
+e.g. 
+
+```python
+def my_function():
+    return 1
+
+print(my_function())
+```
+
+### Many return values
+
+```python
+def my_function():
+    return 2, "A", 79, 3.1314
+
+
+print(my_function()) # -> (2, 'A', 79, 3.1314)
+abcd = my_function() # -> (2, 'A', 79, 3.1314)
+print(abcd) # -> (2, 'A', 79, 3.1314)
+
+a, b, c, d = my_function()
+
+print(a) # -> 2
+print(b) # -> A
+print(c) # -> 79
+print(d) # -> 3.1314
+
+a, b, c = my_function() # ValueError: too many values to unpack (expected 3)
+```
+## [Functions are objects](https://docs.python.org/3/tutorial/controlflow.html#defining-functions)
+
+Functions are objects. Without **()** at the end, you interact with the function. With the **()** you use the function: 
+
+```python
+def my_function():
+    return 1
+
+print(my_function)  # -> <function my_function at 0x7f956166a520>
+print(my_function())  # -> 1
+```
+
+Interesting, but why do I need to know this? Good question, imaginary person! We can use functions as "parameters" via arguments for other functions: 
+
+```python
+def my_function_0():
+    return 3
+
+def my_function_1():
+    return 2
+
+
+def my_function_2(func):
+    return func() ** 2
+
+print(my_function_2(my_function_0))  # -> 9
+print(my_function_2(my_function_1))  # -> 4
+```
+
+## [Default Argument Values](https://docs.python.org/3/tutorial/controlflow.html#default-argument-values)
+
+We can define a default value for arguments: 
+
+```python
+def my_function(a=2, b=7, c=5, d=8):
+    return a + b**2 + c**3 + d**4
+
+print(my_function()) # -> 4272
+```
+
+We can (partially) overwrite the default values: 
+
+```python
+def my_function(a=2, b=7, c=5, d=8):
+    return a + b**2 + c**3 + d**4
+
+print(my_function(b=2,a=3)) # -> 4228
+print(my_function(3,2)) # -> 4228
+```
+
+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.
+
+## Arguments and Keywords
+
+