An Introduction to python Lecture 03: Functionsekprwolf/teaching/... · Note: This is in contrast...

transcript

Priv. Doz. Dr. Roger Wolf http://ekpwww.physik.unikarlsruhe.de/~rwolf/

INSTITUTE OF EXPERIMENTAL PARTICLE PHYSICS (IEKP) – PHYSICS FACULTY

An Introduction to python

Lecture 03: Functions

Roger Wolf23. Mai 2018

Part 1:

The python function

The python function2/20

● Functions in python are defined as shown in the example below:

defines a function

function name

defines a function

function name

function arguments

max(x,y) can have a return value

Here the function is called

● Note: the return value is not type bound! Also the arguments are not strictly tested for their type. As long as the types of both arguments provide a “<” operator in our example the function will give a return value.

● Note: This is in contrast to C++, which is statically typing (see Lecture01). Something like function overloading, like in C++ is not necessary in python.

Checking types3/20

● Still python safes type information that can be checked:

Checking types3/20

You can check the type of each variable using the function type()

● Still python safes type information that can be checked:

The lambda function4/20

● In python you can also define functions at runtime, which are not bound to a name and can 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 any use of a name. The second set of braces passes the argument

Part 2:

Global vs. local variables

Global vs. local variables6/20

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

glob is copied into x here

x changes its value from 10 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 variable are “shadowed” by local scopes, i.e. glob is not visible inside fn().

Another difference to C++7/20

● Note: different from C++ if you define a variable e.g. in a statement block of an if-statement, this variable will still be accessible after that statement block has been left.

Another difference to C++7/20

● Note: different from C++ if you define a variable e.g. in a statement block of an if-statement, this variable will still be accessible after that statement block has been left. j is still

accessible after the ifTry the same for i=0.

References to global variables in functions8/20

● What if you want a reference to a global variable that you need to modify inside a function:

Trial-1: – the naive approach –

Just reference glob in fn()

Trial-2: – pass as function argument –

Pass as argument to fn()

Trial-3: – using the global declaration –

Declare glob as global variable in fn()

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

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

Change glob upon return from fn()

A word on the global declaration10/20

● If you can copy variables into function arguments and again via return, why would you need the global declaration at all?

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

A word on the global declaration10/20

● If you can copy variables into function arguments and again via return, why would you need the global declaration at all?

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

● Note: this discussion is related to an extended discussion about “call by value” and “call by reference” in C(++) (see e.g. An Introduction to C++ Lecture01).

Part 3:

Function arguments

Default arguments12/20

● You can have default arguments for a function:

Default argument

● Note: like in C++ default arguments have to be given at the end of the argument list (see An Introduction to C++ Lecture04).

Default argument

Keyword arguments13/20

● You can refer to the arguments by their names during function call:

default arguments

Calling the function with keyword arguments

keyword arguments

Calling the function with keyword arguments

Part 4:

Function documentation

The DocString15/20

● A string in the first logical line of a function is identified as documentation string (DocString):

The DocString15/20

By convention people use as a DocString a multi-line string starting the first line with a capital letter and ending with a dot. This is followed by an empty line and more detailed explanations following…

The DocString15/20

By convention people use as a DocString a multi-line string starting the first line with a capital letter and ending with a dot. This is followed by an empty line and more detailed explanations following…

● Note: this works equally well for modules (see Lecture04) and classes (see Lecture06)

The DocString16/20

● You can access the DocString via the __doc__ attribute belonging the function:

The DocString16/20

Access the DocString

The DocString16/20

● Note: each module, class or function has the __doc__ field added to it by python automatically. How can this be achieved? – modules, classes and function are themselves objects of classes that derive from base classes, see Lecture06 and 07).

The DocString17/20

● The DocString is also retrieved from the built-in help() function of python. Try the following with the function given in this example:

● Save it to a file max.py● Execute it with python i max.py # this keeps python open after execution● In the prompt type help(max)

The DocString17/20

● The DocString is also retrieved from the built-in help() function of python. Try the following with the function given in this example:

● Save it to a file max.py● Execute it with python i max.py # this keeps python open after execution● In the prompt type help(max)

You should see an output similar to this one:

Part 5:

Variable arguments

Variable arguments19/20

● Python supports passing of an arbitrary number of arguments. These arguments are passed on to the function as tuple or dict:

atuple_of_args will be passed as a tuple

adict_of_args will be passed as a dict

● This feature is known as VarArg parameters.

● Sneak into Lecture05 to learn all you need about tuples and dicts.

Summary

● Functions in python.

● Global sv. local variables.

● Default arguments in functions.

● Keyword arguments in functions.

● The DocString.

● The lambda function.

● VarArgs.

An Introduction to python Lecture 03: Functionsekprwolf/teaching/... · Note: This is in contrast...

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