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Python Cheatsheet

Unless specified otherwise, the following is written for Python 3.

General

  • Python is indentation-based language i.e. indentation defines scope
  • Paramenters in Python function are passed by reference so modifying their attributes within the function will change them.

Running Python

python some_code.py               # interpets python code

Module option

Python has a -m command-line flag which when used will import a module or package for you, then run it as a script

python -m http.server 8080        # runs python localhost:8080 using http.server module script
python -m -v doctest filename.py  # doctests are a quick way of writing tests (v: verbose)

When you don't use the -m flag in regular scenarios, the file you named is run as just a script.

Shebang

See here for the purpose of the shebang (#!) in Unix systems. You may include the shebang at the top of a Python script to execute it without having to type python before the code name when running the code from the terminal. After proper configuration of the file manager, shebang also allows for running a python script via double-clicking its file within the file-manager interface.

For latest version of Python 3 scripts:

#!/usr/bin/env python3

For latest version of Python 2 scripts:

#!/usr/bin/env python2

For both Python 2 and 3 scripts (NOT RECOMMENDED):

#!/usr/bin/env python

Comments

# Single line comment
"""
Multiple
Line
Comment
"""

Semicolon

Unlike other languages such as C, C++, Java, Javascript etc. Python does not require semi-colons to terminate statements. Semicolons are instead used to delimit statements if you wish to put multiple statements on the same line. For example:

var_1 = 1; var_2 = 2; var_3 = 3;

Continuation charater

\ is the continuation character. The line following the continuation character is considered a continuation of the current line:

print("hello" + \
  "word")

if x not in range(8) or \
  y not in range(3):

Main method

def main():
  # do something
  
if __name__ == "__main__": main()

I/O

File

Opening and closing

Python uses the following modes when invoking the open() function:

  • r: read (default)
  • rb: read bytes
  • w: write
  • wb: write bytes
  • r+: read and write

File objects contain a special pair of built-in methods: __enter__() and __exit__(). When a file object's __exit__() method is invoked, it automatically closes the file. Files are typically opened and closed with the following commands:

some_file = open(some_file_path, mode)  # opening a file
some_file.close()                       # closing a file
some_file.closed                        # check if a file is closed

The opening and closing of a file can be conveniently handled with a with-as clause:

with open(some_file_path, mode) as some_file:
  # do something with some_file

Note: some_file is automatically closed when the with-as clause upon completion of the containing block. It is also closed when a return is made from within the block.

Reading

Read the entire file as a string (or otherwise)

with open(some_file_path, mode) as some_file:
  file_contents = some_file.read()

Read the file line by line:

with open(some_file_path, read_mode) as some_file:
  for line in some_file:
    # do something with line

Writing

contents = 'something'
with open(some_file_path, write_mode) as some_file:
  some_file.write(contents)

Note: During the I/O process, data is buffered. this means that it is held in a temporary location before being written to the file. Python doesn't flush the buffer—that is, write data to the file—until it's sure you're done writing One way to do this is to close the file. If you write to a file without closing, the data won't make it to the target file.

Printing

print("Welcome to Python!")         # ends with newline
print("Welcome to Python!", end='') # ends without the default ending newline
print(str1, str2)                   # prints the 2 strings separated by a space
print(lst)                          # prints an list

User input

See here for more info.

Python 2

name = raw_input("What is your name?")  # read in string
num = int(raw_input("give a number"))   # read in number

Python 3

name = input("What is your name?")  # read in string
num = float(input("give a number")) # read in number

Selection statement

if condition_1:
  # do something 1
elif condition_2:
  # do something 2
else:
  # do something else

Note: There are no switch statements in Python

Conditional assignment

x = 10 if a > b else 11

This is analogous to the more common conditional assigment syntax in other languages:

x = a > b ? 10 : 11

Built-in Types

Check types

type('spam')    #=> <type 'str'>
type(1) is int  #=> True

Typecasting

int('1010', 2)  #=> 2     # cast from binary to decimal int
str(2)          #=> "2"   # cast to string
float(2)        #=> 2.0   # cast int 2 into float

Binary

bin(10)       #=> 0b1010
10 == 0b1010  #=> True

Boolean

myBool = True

Operations

Boolean

x and y # CONJUNCTION
x or y  # DISJUNCTION
not x   # NEGATION

Bitwise

Note that they return in decimal format

x | y   # bitwise OR of x and y
x ^ y   # bitwise EXCLUSIVE OR of x and y 
x & y   # bitwise AND of x and y
~x      # NOT of the bits of x
x << n  # x shifted left by n bits
x >> n  # x shifted right by n bits

Numeric

x + y   # sum of x and y
x - y   # difference of x and y
x * y   # product of x and y
x / y   # quotient of x and y
x // y  # floored quotient of x and y
x % y   # remainder of x / y
-x      # x negated
+x      # x unchanged
x ** y  # x to the power y

See PEP 238 regarding floor division operator.

Math

Python contains useful inbuilt math functions

a = [2,3,4,5]
max(a)        #=> 5
min(a)        #=> 2
sum(a)        #=> 14
abs(-1)       #=> 1
pow(x, y)     #=> x ** y
divmod(x, y)  #=> (x // y, x % y)
complex(re, im)   # a complex number with real part re, imaginary part im. im defaults to zero.
c.conjugate()     # conjugate of the complex number c

Strings

s = "String"              # single-line string
s = '''This is a
  mutli-line string'''
s[0]        #=> 'S'       # retrieve character
s[1:4]      #=> 'tri'     # slice
len(s)      #=> 6         # string length
s.lower()   #=> "string"  # downcase
s.upper()   #=> "STRING"  # upcase
s.isalpha() #=> True      # Alpha numeric check 
s.startswith('Str') #=> True
s.endswith('ing')   #=> True
'hello ' + 'world'  #=> 'hello world' # concatenation
','.join('abc')           #=> 'a,b,c'
','.join(['a', 'b', 'c']) #=> 'a,b,c' # For this to work, the argument has to be a list of strings

Formatting

Old style not recommended:

'%s %s' % ('one', 'two')      #=> 'one two'
'%d %d' % (1, 2)              #=> '1 2'
'%2.2f%%' % (0.93788 * 100)   #=> '93.79%'

New recommended style:

'{} {}'.format('one', 'two')  #=> 'one two'
'{} {}'.format(1, 2)          #=> '1 2'
'{0} {0} {1}'.format(1, 2)    #=> '1 1 2'

Loops

For loop

for i in some_list:
  # do something

For-else loop

The else block will execute only when the loop condition is evaluated to False. i.e. it will only run once at the end

for number in numbers: 
  # do something
else:
  # do something else

While loop

while condition:
  # do something

While-else loop

Just like for-else loops, the else block is executed only when the while-loops terminates formally

while condition: 
  # do something
else:
  # do something else

Control flow

break     # breaks out of current loop
continue  # continues to next iteration in loop

Data Structures

These are some common data structures used in Python

List

In python, lists are variable length arrays much like ArrayList in C++ and Vector in Java. Lists offer random access and has similar time complexities (see here) as variable length arrays in other languages. For the sake of consistency, we shall reserve the variable lst to refer to a list in the given context.

Declaration and initialization

Single-dimensional list:
lst = [5, 6, 7, 8]            #=> [5, 6, 7, 8] # explicit assignment
lst = [1] * 4                 #=> [1, 1, 1, 1] # generative assignment
lst = list(range(5,9))        #=> [5, 6, 7, 8] # generative assignment using range
lst = [x for x in range(5,9)] #=> [5, 6, 7, 8] # generative assignment using range and list comprehension (see list comprehension section below)
Multi-dimensional lists:
# Explicit assignment
lst = [[1,2,3],[4,5,6]] #=> [[0, 0], [0, 0], [0, 0]] 

# Generative assignment
cols = 2; rows = 3
lst = [[0] * cols for i in range(rows)] #=> [[0, 0], [0, 0], [0, 0]]

Caveat (see here):

cols = 2; rows = 3
lst = [[0] * cols] * rows #=> [[0, 0], [0, 0], [0, 0]]
lst[0][1] = 1
print(lst)                #=> [[0, 1], [0, 1], [0, 1]]

Range

range(start, stop, step) provides a convenient construct for generating an ordered list of numbers

range(6)            #=> [0,1,2,3,4,5]
range(1,6)          #=> [1,2,3,4,5]
range(1,6,3)        #=> [1,4]
range(10,-1,-1)     #=> [10,9,8,7,6,5,4,3,2,1,0]

3 in range(1,4)     #=> True
3 not in range(1,4) #=> False

Membership

lst.count(item)         # counts the number of times item appears in list 
lst.index(item)         # returns index of first occurence of the item
lst.insert(index, item) # inserts item at position index, suceeding elements are pushed down | in-place | does not return
lst.append(item)        # append item to end of list | in-place | does not return
lst.remove(item)        # remove item | in-place | does not return
lst.pop(index)          # removes object at index and returns it (if not index provided, will pop last) | in-place | returns popped item
del(lst[index])         # same as pop exept it doesn't return the object
del lst[index]          # same as above

Slicing

Slicing is achieved with the [start:end:stride] operator

  • start describes where the slice starts (inclusive)
    • If not provided, value defaults to 0
  • end is where it ends (exclusive)
    • If not provided, value defaults to length of list. I.E. Operation defaults to [start:length:1]
    • If value is negative, Python interpreter will subtract that value off the length of list. For example a[start:-x] is translated to lst[start:length-x]
  • stride indicates the interval between items in the sliced list
    • If not provided, value defaults to 1. I.E. Operation defaults to [start:end:1]
    • start and end defines the sequence for stride to traverse
    • stride indicates the interval in sequence for which objects are picked. For example, a stride of 2 will skip 1 item in sequence at every interval.
    • a positive stride length traverses the sequence in sequential order
    • a negative stride traverses the sequence in reverse order
lst = [0, 1, 2, 3, 4, 5, 6, 7, 8 ,9]
lst[:2]   #=> [0, 1]
lst[3:6]  #=> [2, 3, 4]
lst[:-5]  #=> [0 ,1, 2, 3, 4] # Python interpreter translates to lst[0:5]
lst[::2]  #=> [0, 2, 4, 6, 8]
lst[::-1] #=> [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

Concatenating

lst1 = [1, 2, 3]
lst2 = [4, 5, 6]
lst1 + lst2       #=> [1, 2, 3, 4, 5, 6]

List comprehension

List comprehension is commonly used as a shorthand for creating a new list by iterating over a given list.

It is commonly used as a filter by applying a condition over the given list:

a = [1, 2, 3, 4]
odd_a = [x for x in a if i%2 == 0] #=> [2, 4]

It is also commonly used for modifying each item in the given list:

a = [1, 2, 3, 4]
squared_a = [x**2 for x in a] #=> [1, 4, 9, 16]
squared_even_a = [x**2 if x%2 == 0 else x for x in a] #=> [1, 4, 3, 16]

We can also nest lists within the comprehension to iterate through multi-dimensional lists. One application for this is to flatten out a multi-dimensional list:

lst_2d = [[1, 2, 3], [4, 5, 6]]
flattened_lst = [x for row in lst_2d for x in row ] #=> [1, 2, 3, 4, 5, 6]

Iterating

To iterate over all items in the list:

for item in lst:
  # do something with item

To iterate over all indices for items in the list:

for index in range(len(lst)):
  # do something with index

To iterate over all indicies and their accompanying items in the list:

for index, item in enumerate(lst):
  # do something with index, item

To iterate all items in the list in reversed order (without using slice):

for item in reversed(lst):
  # do something with item

use zip to iterate over multiple lists at once (terminating on the shorter list):

for item_1, item_2, item_3 in zip(lst_1, lst_2, lst_3):
  # do something with item_1, item_2, item_3

Sorting

lst.sort()                          # in-place sorting
tuples = [(5,99), (4,54), (3,12), (2,44)]
sorted(tuples)                      # returns (2,44), (3,12), (4,54), (5,99)
sorted(tuples, key=lambda x: x[1])  # returns (3,12), (2,44), (4,54), (5,99)

Dictionary

In Python dictionary is equivalent to hashmap

Declaration and initialization

Declaring an empty dictionary:

dic = {}
dic = dict()  # using contructor

Declaring and initializing with values:

dic = {'key_1' : val_1, 'key_2' : val_2}          # via explicit assignment
dic = dict([('key_1', val_1), ('key_2', val_2)])  # via key,val pair list
dic = dict(key_1=val_1, key_2=val_2)              # via keyword arguments
dic = {x: x**2 for x in (1, 2, 3)}                # via dict comprehensions

Membership

Common methods to check and update members

key in dic        # check if key is in dic
dic[key] = value  # adding/updating value. Note key can be a number
del dic[key]      # delete (key, val) from dic

Common methods to extract members:

dic.items()   # returns an list of (key, value) tuples, not in any order
dic.keys()    # returns an list of keys, not in any order
dic.values()  # return an list of values, not in any order

Iterating

# Iterates each key (not guaranteed same order everytime)
for key in hashmap:
  # Do something with key
  
# Iterates each key, value pair
for key, value in hashmap.items():
  # Do something with key, value

Sorting

To get a sorted list of keys:

sorted(hashmap)

Shorthands

dic = {'a': 1, 'b': 2, 'c': 3}
[value for (key, value) in sorted(dic.items())] #=> [1, 2, 3]

Importing

import math                   # import math module
from datetime import datetime # import datetime function from datetime module
from math import *            # import all functions from math module into global namespace

Note of caution for from math import *: If you have a function of your very own named sqrt and you import math, your function is safe: there is your sqrt and there is math.sqrt. However if you do from math import *, you have a problem, namely, two different functions with the exact same name.

To return an list of strings listing all the function names in module:

dir(module)

Common modules

os

import os
os.getcwd()                     # get current working directory
os.chdir("d/jinzhe")            # change directory
os.listdir(".")                 # list files in current working directory
os.path.join(par_dir, rel_path) # join par_dir and rel_path together
os.path.abspath(rel_path)       # get the absolute path give relative path

sys

import sys

Getting arguments:

sys.argv[1] # first argument

Pythonpath stores the list of directories Python scans through when locating modules and files. You may append directories to your Pythonpath via sys.path.append instead of appending to $PYTHONPATH environment variable:

sys.path.append(some_path)

argparse

See here for all available arugments of add_argument() method

import argparse

parser = argparse.ArgumentParser(description="Description of script")
parser.add_argument('--flag', '-f', action='store_true')
parser.add_argument('--input', '-i', type=int, default=1, help='Help text for input argument')
args = parser.parse_args()
args.flag   # gets boolean if flag was indicated
args.input  # gets argument for --input

pickle

Python 2
import cPickle as pkl
loaded_pickle = pkl.load(open(pickle_path, "r" ))   # loading
pkl.dump(payload, open(pickle_path, "w" ))          # dumping
Python 3
import pickle as pkl
loaded_pickle = pkl.load(open(pickle_path, "rb" ))  # loading
pkl.dump(payload, open(pickle_path, "wb" ))         # dumping

defaultdict

from collections import defaultdict

# int type: non-existent keys will have default value 0 when first called
dic = defaultdict(int)
dic[key] += n

# list type: non-existent keys will have default value list() when first called
dic = defaultdict(list)
dic[key].append(n)

# set type: non-existent keys will have default value set() when first called
dic = defaultdict(set)
dic[key].add(n)

namedtuple

TODO

from collections import namedtuple

ordereddict

TODO

from collections import OrderedDict

re

import re
re.search(r'\d+\.\d*N','1.312740N, 103.779490E').group(0) #=> '1.312740N'

datetime

import datetime
date = datetime.now()   # current date and time
date.year
date.month
date.day
date.hour
date.minute
date.second

random

from random import randint
randint(low, high)  # generates random interger between low to high inclusive

enum (only available on Python 3.4 onwards)

from enum import Enum
class EdgeDetector(Enum):
  prewit = 0
  sobel = 1

multiprocessing.Pool

See API

Importing:

from multiprocessing import Pool

Pool:

pool = Pool() # If processes is None then the number returned by multiprocessing.cpu_count()
# do something
pool.close()

# Using `with` clause
with Pool(processes=4) as pool:
  # do something
  • Make sure function is defined before Pool istantiates! Otherwise the worker cannot see the function

Using pool.apply_async:

result_1 = pool.apply_async(f, 1)    # evaluate `f(1)` asynchronously
result_2 = pool.apply_async(f, 2)    # evaluate `f(2)` asynchronously
answer_1 = result_1.get(timeout=10)
answer_2 = result_2.get(timeout=10)

Using pool.map:

result_1, result_2 = pool.map(f, [1,2])
  • This function only works for single argument functions! Use a function wrapper which takes in a single tuple if parallelizing multi argument functions

multiprocessing.Process

from multiprocessing import Pool

def f(arg):
  # do something

args_list = [1,2,3,4]
for arg in args_list:
  p = Process(target=f, args=(arg,))
  p.start()
  p.join()

Functions

The default function return value in Python is None

def some_function(param_1, kwarg_1=default_kwarg_1_val):
  # do something
  return ans

Lambda: anonymous functions

lst = [0, 1, 2, 3, 4, 5, 6]
filter(lambda x: x % 3 == 0, lst) #=> [0, 3, 6]

Class

Declaration

There are multiple ways to declare a class.

Python 3: Delcare a class with implicit inheritance of object class:

class MyClass:
  pass

Python 3: Declare a class with implicit inheritance of object class:

Python 2: Declare an old-style class:

class MyClass():
 pass

Python 3: Declare a class with explicit inheritance of object class:

Python 2: Declare a new-style class:

class MyClass(object):
  pass

See here on old-style and new-style classes

Instantiation

my_class_object = MyClass(attr1, attr2, attr3) # instantiating MyClass

Methods

Constructor

The first argument __init__() gets is used to refer to the instance object by convention, that argument is called self. If you add additional arguments in the body of __init__(), you need to give each instance those attributes. self is only passed as an argument when defining methods, but are left out when calling them.

Class representation

By providing a return value in this method, we can tell Python how to represent an object of our class (for instance, when using a print statement)

__repr__(self):  
  return "(#{}, #{}, #{})" % (self.attr_1, self.attr_2, self.attr_3)

Class example

class MyClass(object):
  # class attributes or "member variables", information that belongs to the class object
  attr_1 = attr_1_val  # Note: must be referenced later via self.attr_1

  # constructor with instance attributes
  def __init__(self, attr1, attr2, attr3):
    self.attr1 = attr1
    self.attr2 = attr2
    self.attr3 = attr3

    # methods (need to pass in self)
    def method_1(self, param_1, param_2):
      # do something

Inheritance example

class MyOtherClass(MyClass):
  # new constructor
  def __init__(self, attr1, attr2, attr3, attr4):
    super(MyOtherClass, self).__init__(attr1, attr2, attr3) # super method (don't have to include self)
    self.attr4 = attr4

  # override method_1 from MyClass
  def method_1(self):
    # do something new

  def old_method_1(self):
    return super(MyOtherClass, self).method_1()  # super method. Note the return statement

kwargs

def func(**kwargs):
  # do stuff with kwargs['a'] and kwargs['b']

func(a=1, b=3)
def func(a, b):
  print(a, b)

func(**{'a': 1, 'b': 2}) # same as func(1, 2)
#=> 1 2

Tuples

Tuples are immutable in Python

uniple = (1,)
pair = (1, 2)
triplet = (1, 2) + (3,) # creates a new tuplet

Misc

len(some_object) # returns length attribute of the object

Exception handling

Common exceptions

Exception Description
IOError Raised when a file cannot be opened
ImportError Raised when python cannot find the module
ValueError Raised when a built-in operation or function receives an argument that has the right type but an inappropriate value
ZeroDivisionError Raised when division by zero occurs
EOFError Raised when one of the built-in functions (input() or raw_input()) hits an end-of-file condition (EOF) without reading any data
KeyboardInterrupt Raised when the user hits the interrupt key (normally Control-C or Delete)
ValueError Raised when a built-in operation or function receives an argument that has the right type but an inappropriate value

See Built-in Exceptions for the full list of Python exceptions that come out of the box.

Catching exceptions

try-except:

try:
  # something dangerous

except IOError:
  # do something
  
except (RuntimeError, TypeError, NameError): # multiple exceptions
  # do something
  
# Generic exception: Catches all exceptions
except Exception as e: # access exception variable with `as e`
  # do something. e.g `print(str(e))`

Else:

try:
  # something dangerous
  
except KeyboardInterrupt:
  # do something

# ONLY WHEN preceding except clauses did not catch anything
else:
  # do something
  # NOTE: Code in this block may raise exceptions of its own

Finally:

try:
  # something dangerous

except KeyboardInterrupt:
  # do something

else:
  # do something

# Always executed before leaving the try statement, regardless if an exception occurred or not
finally:
  # do something

Raising exceptions

Raising pre-defined exceptions:

raise IOError
raise SyntaxError("Oops!") # Add a custom message

Raising custom exception:

class MyException(Exception):
  pass

raise MyException("Truly exceptional!")