Python Cheat Sheet for Data Science: Basics

It's common when first learning Python for Data Science to have trouble remembering all the syntax that you need. 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 Intermediate 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

x = 3 | Assign 3 to the variable x
print(x) | Print the value of x
type(x) | Return the type of the variable x (in this case, int for integer)
help(x) | Show documentation for the str data type
help(print) | Show documentation for the print() function

Reading Files

f = open("my_file.txt", "r")
file_as_string = f.read()

Open the file my_file.txt and assign its contents to string

import csv
f = open("my_dataset.csv", "r")
csvreader = csv.reader(f)
csv_as_list = list(csvreader)

Open the CSV file my_dataset.csv and assign its data to the list of lists csv_as_list

Strings

s = "hello" | Assign the string "hello" to the variable s

s = """
She said,"there's a good idea.
""""

Assign a multi-line string to the variable s. Also used to create strings that contain both" and ' characters.

len(s) | Return the number of characters in s
s.startswith("hel") | Test whether s starts with the substring "hel"
s.endswith("lo") | Test whether s ends with the substring "lo"
"{} plus {} is {}".format(3,1,4) | Return the string with the values 3, 1, and 4 inserted
s.replace("e","z") | Return a new string based on s with all occurrences of "e" replaced with "z"
s.strip() | Return a new string based off s with any whitespace at the start and end of the string removed
s.split(" ") | Split the string s into a list of strings, separating on the character " " and return that list

Numeric Types and Mathematical Operations

i = int("5") | Convert the string "5" to the integer 5 and assign the result to i
f = float("2.5") | Convert the string "2.5" to the float value 2.5 and assign the result to f
5 + 5 | Addition
5 - 5 | Subtraction
10 / 2 | Division
5 * 2 | Multiplication
3 2 | Raise 3 to the power of 2 (or (3^{2}))
27 (1/3) | The 3rd root of 27 (or (\sqrt[3]{27}))
x += 1 | Assign the value of x + 1 to x
x -= 1 | Assign the value of x - 1 to x

Lists

l = [100, 21, 88, 3] | Assign a list containing the integers 100, 21, 88, and 3 to the variable l
l = list() | Create an empty list and assign the result to l
l[0] | Return the first value in the list l
l[-1] | Return the last value in the list l
l[1:3] | Return a slice (list) containing the second and third values of l
len(l) | Return the number of elements in l
sum(l) | Return the sum of the values of l
min(l) | Return the minimum value from l
max(l) | Return the maximum value from l
l.append(16) | Append the value 16 to the end of l
l.sort() | Sort the items in l in ascending order
" ".join(["A", "B", "C", "D"]) | Converts the list ["A", "B", "C", "D"] into the string "A B C D"

Dictionaries

d = {"CA": "Canada", "GB": "Great Britain", "IN": "India"} | Create a dictionary with keys of "CA", "GB", and "IN" and corresponding values of of "Canada", "Great Britain", and "India"
d["GB"] | Return the value from the dictionary d that has the key "GB"
d.get("AU","Sorry") | Return the value from the dictionary d that has the key "AU", or the string "Sorry" if the key "AU" is not found in d
d.keys() | Return a list of the keys from d
d.values() | Return a list of the values from d
d.items() | Return a list of (key, value) pairs from d

Modules and Functions

The body of a function is defined through indentation

import random | Import the module random
from random import random | Import the function random from the module random

def calculate(addition_one,addition_two,exponent=1,factor=1):
    result = (value_one + value_two) ** exponent * factor
    return result

Define a new function calculate with two required and two optional named arguments which calculates and returns a result.

addition(3,5,factor=10) | Run the addition function with the values 3 and 5 and the named argument 10

Boolean Comparisons

x == 5 | Test whether x is equal to 5
x != 5 | Test whether x is not equal to 5
x > 5 | Test whether x is greater than 5
x < 5 | Test whether x is less than 5
x >= 5 | Test whether x is greater than or equal to 5
x <= 5 | Test whether x is less than or equal to 5
x == 5 or name == "alfred" | Test whether x is equal to 5 or name is equal to "alfred"
x == 5 and name == "alfred" | Test whether x is equal to 5 and name is equal to "alfred"
5 in l | Checks whether the value 5 exists in the list l
"GB" in d | Checks whether the value "GB" exists in the keys for d

Statements and Loops

The body of if statements and loops are defined through indentation

if x > 5:
    print("{} is greater than five".format(x))
elif x < 0:
    print("{} is negative".format(x))
else:
    print("{} is between zero and five".format(x))

Test the value of the variable x and run the code body based on the value

for value in l:
    print(value)

Iterate over each value in l, running the code in the body of the loop with each iteration.

while x < 10:
    x += 1

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

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