Python:Control structures

Most flow control is standard. Indentation defines the code block to execute. You must be strict, all indents must be the same in a block, a mix of spaces and tabs is not allowed (unless each line uses the same mix).

You can define you own Python:Functions too.

if expression:
    block
elif expression:
    block
else:
    otherblock

for var1 in itterable:  # keys if itterable is a dict
    block
    if somethinghappens:
        break

while expression:
    block
    if somethinghappens:
        continue

break

Stop looping immediately. Jump to the next statement after the loop.

continue

Stop the current iteration immediately and jump the the top of the loop, do the next iteration.

pass

No operation, use e.g. to avoid negative tests

var1 = ( x if <condition1> elif <condition2> y else z )

Different way to assign a value conditionally. () only needed if spread over more lines.

Exception handling

Exceptions are used a lot by python programmers. The mantra is: "Easier to ask for forgiveness than permission" aka EAFP; and it is said to be faster.

To catch exceptions:

try:
    blockthatmaythowexception
except <exception>:
    blockifexceptionisthrown
except <exception2> as ex:
    blockifexception2isthrown
else:
    blockif_NO_exceptionisthrown

NOTE: elif is not allowed here.

Specifying a specific exception is optional but highly recommended. However, take care you don't forgive real faults.

If more exceptions are defined with 1 try only 1 exception block will be executed,

For reporting where the exception happened use #traceback.

except <exception> as ex

Puts the exception result in ex. ex.message has the reason for the exception.

type(ex)

The exception class

except Exception

Catches any exception

except NameError

The variable does not exist

except KeyError

The key does not exist in the dict

except IndexError

The index does not exist in the list

A full list of exception-classes can be found in the python docs

raise <Exception>(<text>)

Raise an <Exception> exception, <Exception>-class must exist, <text> is up to you.

Traceback

Show where things went wrong

import traceback
tracetext = traceback.format_exc()   # Return info as string
traceback.print_exc()  # Print the info

Warnings

Warnings are a subcategory of Exceptions for problems that do not break the script. E.g. for deprecated modules, division by 0 etc.

After importing the warnings package you can do things like

warnings.simplefilter(action='ignore', category=UserWarning)

Ignore warnings in a specific category.

warnings.filterwarnings("error")

Turn warnings into exceptions

warnings.resetwarnings()

Reset to the default handling of warnings

Example:

import warnings
...
...
warnings.simplefilter(action='ignore', category=UserWarning)
<code that throws warning>
warnings.resetwarnings()

This way the warning settings are restored after the known one has been passed