Python Cheat Sheet for Data Science: Intermediate

The printable version of this cheat sheet

The tough thing about learning data is remembering all the syntax. While at Dataquest we advocate getting used to consulting the Python documentation, sometimes it's nice to have a handy reference, so we've put together this cheat sheet to help you out!

This cheat sheet is the companion to our Python Basics Data Science Cheat Sheet

If you'd like to learn Python, we have a Python Programming: Beginner course which can start you on your data science journey.

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Key Basics, Printing and Getting Help

This cheat sheet assumes you are familiar with the content of our Python Basics Cheat Sheet.

s | A Python string variable
i | A Python integer variable
f | A Python float variable
l | A Python list variable
d | A Python dictionary variable

Lists

l.pop(3) | Returns the fourth item from l and deletes it from the list
l.remove(x) | Removes the first item in l that is equal to x
l.reverse() | Reverses the order of the items in l
l[1::2] | Returns every second item from l, commencing from the 1st item
l[-5:] | Returns the last 5 items from l

Strings

s.lower() | Returns a lowercase version of s
s.title() | Returns s with the first letter of every word capitalized
"23".zfill(4) | Returns "0023" by left-filling the string with 0's to make it's length 4.
s.splitlines() | Returns a list by splitting the string on any newline characters.
Python strings share some common methods with lists|
s[:5] | Returns the first 5 characters of s
"fri" + "end" | Returns "friend"
"end" in s | Returns True if the substring "end" is found in s

Range

Range objects are useful for creating sequences of integers for looping.

range(5) | Returns a sequence from 0 to 4
range(2000,2018) | Returns a sequence from 2000 to 2017
range(0,11,2) | Returns a sequence from 0 to 10, with each item incrementing by 2
range(0,-10,-1) | Returns a sequence from 0 to -9
list(range(5)) | Returns a list from 0 to 4

Dictionaries

max(d, key=d.get) | Return the key that corresponds to the largest value in d
min(d, key=d.get) | Return the key that corresponds to the smallest value in d

Sets

my_set = set(l) | Returns a set object containing the unique values from l
len(my_set) | Returns the number of objects in my_set (or, the number of unique values from l)
a in my_set | Returns True if the value a exists in my_set

Regular expressions

import re | Import the Regular Expressions module
re.search("abc",s) | Returns a match object if the regex "abc" is found in s, otherwise None
re.sub("abc","xyz",s) | Returns a string where all instances matching regex "abc" are replaced by "xyz"

List comprehension

A one-line expression of a for loop

[i ** 2 for i in range(10)] | Returns a list of the squares of values from 0 to 9
[s.lower() for s in l_strings] | Returns the list l_strings, with each item having had the .lower() method applied
[i for i in l_floats if i < 0.5] | Returns the items from l_floats that are less than 0.5

Functions for looping

for i, value in enumerate(l):
    print("The value of item {} is {}".format(i,value))

Iterates over the list l, printing the index location of each item and its value

for one, two in zip(l_one,l_two):
    print("one: {}, two: {}".format(one,two))

Iterates over two lists, l_one and l_two and print each value

while x < 10:
    x += 1

Runs the code in the body of the loop until the value of x is no longer less than 10

Datetime

import datetime as dt | Imports the datetime module
now = dt.datetime.now() | Assigns datetime object representing the current time to now
wks4 = dt.datetime.timedelta(weeks=4) | Assigns a timedelta object representing a timespan of 4 weeks to wks4
now - wks4 | Returns a datetime object representing the time 4 weeks prior to now
newyear_2020 = dt.datetime(year=2020, month=12, day=31) | Assigns a datetime object representing December 25, 2020 to newyear_2020
newyear_2020.strftime("%A, %b %d, %Y") | Returns "Thursday, Dec 31, 2020"
dt.datetime.strptime('Dec 31, 2020',"%b %d, %Y") | Returns a datetime object representing December 31, 2020

Random

import random | Imports the random module
random.random() | Returns a random float between 0.0 and 1.0
random.randint(0,10) | Returns a random integer between 0 and 10
random.choice(l) | Returns a random item from the list l

Counter

from collections import Counter | Imports the Counter class
c = Counter(l) | Assigns a Counter (dict-like) object with the counts of each unique item from l, to c
c.most_common(3) | Returns the 3 most common items from l

Try/Except

Catch and deal with errors

l_ints = [1, 2, 3, "", 5]

Assigns a list of integers with one missing value to l_ints

l_floats = []
for i in l_ints:
    try:
        l_floats.append(float(i))
    except:
        l_floats.append(i)

Converts each value of l_ints to a float, catching and handling ValueError: could not convert string to float: where values are missing.

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