An Introduction to python - ekprwolf/teaching/courses/Python201… · In python functions are...

Priv. Doz. Dr. Roger Wolf http://ekpwww.physik.uni-karlsruhe.de/~rwolf/

INSTITUTE OF EXPERIMENTAL PARTICLE PHYSICS (IEKP) – PHYSICS FACULTY

An Introduction to python

Lecture 05: Functions

Roger Wolf06. June 2019

Part 11:

The python function

Function definition2/25

● In python functions are defined as shown in the example below:

Defines a function.

Function definition

Defines a function.

Function name.

Function definition

Defines a function.

Function name.

Function arguments.

Function definition

Interface & implementation

Function interface to outside world/user.

Implementation of the function.

Implementation of function ends here.

● The function opens its own range of validity (=statement block). I.e. everything indented after the function definition is part of the function.

● Again note the importance of indentation in this case.

Function call4/25

● The following example shows how the function is called inside your script:

Function call4/25

● The following example shows how the function is called inside your script:

Here the function is called.

● Function arguments are given in braces, separated by comma.

Return value5/25

● A function may have a return value (while it does not have to):

Return value5/25

max(x,y) has a return value.

● The return value is indicated by the key word return.

Return value5/25

● The return value is not type bound, i.e. it could be an int, float, string, tuple, list, … depending on the context (→ see the example above).

Return value5/25

● The return value is not type bound, i.e. it could be an int, float, string, tuple, list, … depending on the context (→ see the example above).

● If there is nothing returned the return value will be None.

Types of function arguments6/25

● The following is a dangerous example of using dynamic typing:

The result is 6.

The result is aaa.

The result is 6.

The result is aaa.

● As you see the function does different things depending on the function arguments at runtime. This is possible as long as an operator (here “*”) is defined for all types in use.

● If you have to be sure about the type of your arguments you can always check it using the function type.

Checking the type of a variable exploiting the type function.

● Tuples (of arbitrary length) can easily be passed to or returned from functions:

Tuples & functions

Expects two tuples of types (float, float).

Tuples & functions

Expects two tuples of types (float, float).

Also returns a tuple of type (float, float).

The lambda function10/25

● You can define functions at runtime, which are not bound to a name. These functions are usually used to fulfill simple tasks:

This is a normal (named) function.

This is a (nameless) lambda function, g is a pointer to this function.

This is the same lambda function w/o name nor function pointer. The first set of braces defines the function, the second set of braces passes the arguments.

Built-in (common) functions to be applied on sequences11/25

● You can apply several useful operations on all sequences, among those:

● len(seq):

Return the length of seq.

● filter(fn, seq):

Apply a function fn to each element of seq; fn should return a boolean; filter returns an iterator through the elements of seq for which fn is true.

● map(fn, seq):

Apply a function fn to each element of seq; fn gives a return value for each element; map returns an iterator through the elements of seq to which fn has been applied element-wise.

● reduce(fn, seq):

Apply a function to each element of seq; fn gives a rule how to combine two elements of seq; reduce returns a combination of all elements applying fn pair-wise up to the end of the sequence.

● Task: Create a list of sorted objects that can be divided by three w/o rest class from an arbitrary input list:

Example for the built-in function filter

● Task: Create a list of sorted objects that can be divided by three w/o rest class from an arbitrary input list:

Example for the built-in function filter

● The function filter is used with a lambda function that returns a boolean.

● The return value of filter is an iterator through the elements in foo, for which the lambda function returns true.

● To turn these into a new list we use list comprehension.

Example for the built-in function map

● Task: Create a dictionary from the input, with all string values in lower case:

Iterator to the key value pairs in moo.

Lambda function takes key value pair as input & returns a key value pair w/ string value in lower case.

Lambda function takes key value pair as input & returns a key value pair w/ string value in lower case.Access actual key value

pair from iterator.

Lambda function takes key value pair as input & returns a key value pair w/ string value in lower case.Access actual key value

pair from iterator.

● Note: This is not the most elegant way of doing this…

Thinking python: … doing a very similar thing in one line of code. Can you decipher what is being done here? Can you see the difference in output to what has been discussed before?

● Task: Calculate the magnitude of a vector of any dimension, represented by a tuple.

Example for the common function reduce

Example for a lambda function w/ two arg’s.

● The lambda function gives the rule, how to combine two elements of the sequence, pairwise. In this case the return value of reduce is a single number, to which the sequence has been reduced.

Part 13:

Global vs. local variables

Global vs. local variables17/25

● Per default function arguments are passed by value, i.e. a variable that is defined in a global scope can only be changed locally, i.e. inside a function:

The value of each function argument is copied into x here.

x changes its value to 5 here.

But glob kept its initial value.

● Local variables, which are defined in fn, will be freed and are not accessible any more after fn is left.

● Global variables are “shadowed” by local scopes, i.e. glob is not visible inside fn.

Side remark19/25

● The statement at the bottom does not apply to the scope of if statements, as the following example shows:

Side remark19/25

j defined inside statement block of if statement.

Side remark19/25

But still accessible outside.

Side remark19/25

Try the same for i=0.

References to global variables in functions20/25

● Task: Try to access/modify a global variable inside a function:

Trial-1: – the naive approach –

Access glob, which is defined in the scope above fn.

Trial-2: – pass as function argument –

Pass as function argument here.

Trial-3: – using the global declaration –

Make glob known as global variable inside fn using the key word global.

● Note: you can declare more than one global variable with one global statement, e.g.: global x,y,z.

Change glob upon return value of fn.

Trial-4: – and don’t forget the obvious –

A word on the global declaration24/25

● If you can copy variables into function arguments and back via return values, what do you need the global declaration for at all?

A word on the global declaration24/25

● If you can copy variables into function arguments and back via return values, what do you need the global declaration for at all?

● Imagine glob being not a simple number, but a many GB huge object that cannot be copied around w/o blocking your active memory. In this case you cannot pass glob as argument, but you have to manipulate it itself. In this situation the global statement becomes vital for you.

Summary25/25

● Part 11: The python function.

● Part 12: Lambda functions.

● Part 13: Global vs. local variables.

An Introduction to python - ekprwolf/teaching/courses/Python201… · In python functions are...

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