Python Cheat Sheet

1 * 5

2 / 5

2 ** 5

5 % 3

#characters, use " or ' to contain them
string = "string"

#whole numbers
integer = 5

#real numbers
floats = 12.23

#True, False or None
boolean = True

#similar to an array, any mix of data types
#lists are the most common base python container
lists = ['anymixoftypes', False, 5, 12.3]

#tuples are similar to a list, but immutable - you cannot change their contents
tuples = ("immutable", "lists")

#Dictionaries are similar to a hash table, key:value pairs. Unordered
dictionary = {'key1' : 'value', 'key2' : 34}

#Sets only keep unique elements. Not able to index
sets = set([1,1,2,3,4,5])

a = [1,2,3,4,5]
#python is 0 indexed = 0 is the first element
a[0]

#negative numbers start at the end
a[-1]

#use : to get everything
a[2:]

#nested lists can be subset
a = [1,2,3,[4,5,6],7]
a[3][1]

#dictionaries are accessed by their keys
a = {'key1':12, 'key2':[1,2,3]}
a['key2']

#Modules are imported using 'import'
import numpy as np
import pandas as pd
#now if we want to use a numpy function, we use np.function

#we can import certain functions, so we don't have to use the module name
from pandas import Series, DataFrame
#now Series and Dataframe will work without the pd. first

#an array allows vectorized operations
array = np.array([1,2,3,4,5])
#can only contain one type of data
array - 1

#arrays can be multidimensional
array2d = np.array([[1,2,3,4,5],
                    [6,7,8,9,10]])
#still only one type
array2d - 1

#Series allows 'indexed' data in arrays
series = Series([1,2,3,4,5], index = ['a', 'b', 'c', 'd', 'e'])
series
#still only one data type (index does not count)

#Dataframes are 2d collections of data - like a spreadsheet or SQL table
dataframe = DataFrame(np.array([1,2,3,4]).reshape(2, 2),
                      columns = list('ab'),
                      index = ['X','Y'])
dataframe
#has column names and indexes

#for loops
for i in [1,2,3,4]:
    print(i ** 2)

#while loops
x = 0
while x < 5:
    print('%s is less than 5' %(x))
    x += 1

#if else
a = [1, 2, 3, 4]
for number in a:
    if number == 1:
        print(number)
    elif number == 2:
        print("two")
    else:
        print("I'm not sure")

#shortcut for a for loop returning a list
a = [1,2,3,4,5]
[i * i for i in a]

#to call a function, use its name and arguments
sum([1,2])
#use help(function) to get help on a function
#help(sum)

#to make your own
def mysum(a, b):
    '''
    This is the help (docstring), to see this type help(mysum)
    '''
    return(a + b)

help(mysum)

mysum(1,2)

set([1,2,3]).pop()

#Get help using help(object.method), note no ()
help(set([1,2,3]).pop)

#methods will often modify in place!
a = [5, 4, 3, 2, 1]
a.sort()
a

#arguments go in the braces
a.sort(reverse = True)
a

#define objects using class
#with a way to init a new one
#and any other methods
class myclass:
    '''
    help goes here!
    '''
    def __init__(self, x, y):
        self.x = x
        self.y = y
    def mymethod(self, text):
        print(text)

#make an object
a = myclass(1,2)
a.x, a.y

# call a method
a.mymethod("mytext")