Python Cheatsheet

Table of Contents

• External Cheat Sheets
• 1. Types and Common Operators/Functions
• 2. Strings
• 3. Lists
• 4. Dictionaries
• 5. Conditionals
• 6. Functions
• 7. Loops
• 8. Files

External Cheat Sheets

1. Python 2.7 Quick Reference Sheet
2. Dave Child's Python cheatsheet

Also check out our list of resources here

1. Types and common operators / functions

• a in b: Test whether a is contained in b and returns True if so. b can be a string, list, tuple, set or dictionary.

• a not in b: Returns True ir a is NOT contained in b. b can be a string, list, tuple, set or dictionary.

• type(obj): Returns the data type of an obj.

• str(obj): Returns a string representation of an obj, i.e. obj coerced to a string

• int(obj): Returns an integer representation of an obj, i.e. obj coerced to an integer

• float(obj): Returns a float representation of an obj, i.e. obj coerced to an float

• len(seq): Returns the number of items in a sequence seq: a sequence can be a string, list, tuple, etc.

• range(int): Returns a sequences of integers from 0 to int-1.

• range(start, end, by): Returns a sequences of integers from start to end-1, in steps of by.

> range(4, 10, 2)
4, 6, 8

2. Strings

• string[index1 : index2]: Slices a string from index1 to index2 (excluding index2)

• string.upper(): Makes a string all uppercase.

> s = "donut"
> s.upper()
DONUT

• string.lowercase(): Makes a string all lowercase.

• string.join(list): Concatenates each item in list with the delimiter string.

> l = ["Billy", "Bob", "Thorton"]
> "".join(l)
"BillyBobThorton"
> "--".join(l)
"Billy--Bob--Thorton"

• string1.startswith(string2): Returns True if string1 starts with string2, False otherwise.

• string1.endswith(string2): Returns True if string1 ends with string2, False otherwise.

• substring in string: Returns True if substring is contained in string, False otherwise.

3. Lists

• list[index]: Finds the item in list with the position index.

• list[index1 : index2]: Slices a list from index1 to index2 (excluding index2)

• list.append(obj): Appends obj to the end of list

• list.extend(obj): Extends list to include obj, which is usually another list.

• list.index(obj): Returns index of first occurrence of obj in list; raises ValueError if obj not in list.

• list.remove(obj): Removes first instance of obj in list; raises ValueError if obj not in list.

• list.sort(): Sorts list in order, in place.

• list.count(obj): Returns an integer counting the number of occurrences of obj in list

• obj in list: Returns True if obt is contained in list, False otherwise.

4. Dictionaries

• dict[key]: Returns the value paired with key in dict. key is usually a string. Can also be used in assignment:

> dict = {} # make empty dictionary
> dict['name'] = 'Barack Obama' # assign a key / value pair to `dict`
> dict['name'] # lookup
'Barack Obama'

• key in dict: Returns True if dict contains key, False otherwise.

• dict.keys(): Returns a list of dict's keys.

• dict.values(): Returns a list of dict`'s values.

• dict.items(): Returns a list of key / value pairs in dict`. Each list item is a 2 item tuple.

5. Conditionals

• Conditional syntax:

> if (boolean_exp):
     do something
> elif (boolean_exp):
     do something
> else:
     do another thing

• Boolean operators: and, or, not

> if (boolean_exp) and (boolean_exp):
    do something

6. Functions

• define function

> def function_name(arg1, arg2):
     answer = arg1 + arg2
     return(answer)

• call function

> function_name(2, 3)
5

7. Loops

• for loops:

> for item in list:
      do something

• alter every item in a list, and store those values in another list:

> orig_list = [1, 2, 3] # make list to iterate over
> stored = [] # make empty list to store new values

> for item in orig_list:
      new_item = item + 2 # do something to each item, here we add 2
      stored.append(new_item) # store that new item in the `stored` list.

> stored
[3, 4, 5]

• alter every item in a list and store those values in the same list

> for index in range(len(orig_list)): # iterate over indices
      orig_list[index] = orig_list[index] + 2

> orig_list
[3, 4, 5]

• iterate over indices + values at the same time using multiple iteration

> fruit = ['apples', 'bananas', 'kiwis']

> for val, index in enumerate(fruit):
      print val, index

0 'apples'
1 'bananas'
2 'kiwis'

8. Files

• Read contents of file example.txt into variable text

> with open('example.txt') as my_file:
      text = my_file.read()

• Read in a file line by line, storing those lines as a list

> stored = [] # create empty list to store lines
> with open('example.txt') as my_file:
     for line in my_file:
         stored.append(line.strip()) # line.strip() will get rid of line breaks characters.

• Write a list to a file, with each item on its own line.

bees = ['bears', 'beets', 'Battlestar Galactica']
with open('example.txt', 'w') as new_file:
    for i in bees:
        new_file.write(i + '\n') # will write each item on its own line

• Read in a CSV as a list of dictionaries, with each dictionary representing a row and keys representing columns.

> stored = [] # make empty list to store dictionaries
> with open('example.csv', 'rU') as csvfile: # open file
     reader = csv.DictReader(csvfile) # create a reader
     for row in reader: # loop through rows
         stored.append(row) # append each row to the list

• Write list of dictionaries as a CSV (note that each dictionary must have the same keys!)

> keys = stored[0].keys() # get a list of the keys, which will be column names.
> with open('example.csv', 'wb') as output_file:
     dict_writer = csv.DictWriter(output_file, keys)
     dict_writer.writeheader()
     dict_writer.writerows(stored)