Python 3 - Cheat Sheet

Introduction to PythonVT17

Quick links from the Standard Library


Type Example Convert to type using
None None -
Integer int 0, 1, 100 int()
Float float 0.0, .5, 10.453 float()
String str ‘hello’ str()
List list [1,’hi’,None,3.14] list([iterable])
Tuple tuple [1,’hi’,None,3.14] tuple([iterable])
Dictionary dict { ‘a’ : 1, ‘b’: 2 } dict([iterable])
Set set { ‘a’, ‘b’ } dict([iterable])

Create your own type using class

Iterating through list elements

for item in aList:
	print('List item: ', item)

It is not necessary to iterate list items via their position. You rarely need to do:

for i in range(len(aList)):
	print('List item: ', aList[i])

If you are looking for an element in a list and cannot find it, you can use the else clause. For example, we are searching for the first integer in aList that is bigger than 10.

aList = [ 1, 4, 7, 6, 3, 4, 32, 17, 18, 10, 8]
for item in aList:
	if item > 10:
		return item
	return None

Iterating through a dictionary

for key in dictionary:
	print('Key item: ', key)

Same as

for key in dictionary.keys():
	print('Key item: ', key)

For the values only

for v in dictionary.values():
	print('Value item: ', v)

For both keys and values

for k,v in dictionary.items():
	print('Key item :', k, ' | Value item: ', v)

Read a file line by line

with open("filename","r") as f: # Add encoding='...' if necessary
	for line in f:
		print('Line: ', line.rstrip()) # Removing the trailing newline character
		# print('Line: ', line, end='') # Or that works too

The with statement takes care of closing the file in case an error occurs in the for loop.

Defining a function

def functionName(arg1, arg2, arg3=None, arg4='test.log', arg5=32):
	# body of the function

arg1 and arg2 are positional arguments, required when calling the function.

arg3, arg4, arg5 are keyword arguments with a default value (respectively None, the string test.log, and the interger 32)

Positional arguments are always first, and in order. Keyword arguments are last, can be omitted and/or given in any desired order.

You can call the above function with, for example, functionName(12,3.14,arg5=10,arg4='Blabla.tex').

In the body, at runtime,

Importing a function from a module

from moduleName import functionName

Or if we want to rename locally the function:

from moduleName import functionName as newName

Testing a condition

We need boolean logic to compare values. We have of course < >, <=, >=, to compare integers, floats or even strings. If we compare objects, Python will compare their size (by calling __len__()).

The comparator == and != checks the values, while is checks the id (ie are the same object?). Obviously, when v1 is v2, it implies v1 == v2.

A condition can combine expressions with the above comparators, using the logical and and or operators, and not condition is its negation.

if condition:
	print('Condition is true')
	print('Condition is false')

We can pass an object as the condition. This is very convenient as Python will check if the object is “False”.

False objects are:

If you define your own class, you’ll have to add the function __bool__() (or __len__(), since Python defaults to __len__ when __bool__ is not present)

For example, when calling a function where arguments have default values

def functionName(arg1=None):
	if arg1:
		print('arg1 was None')
	# Body of the function

Catching errors

	# Do something
except ErrorName:
	print("An error of type ErrorName occured")

print('Continue after the error')