Python Syntax Cheat Sheet

To write a comment in python use the hash tag at the start of the line:

# This is a comment.

To print use the keyword print():

print("Hello World!")

Important: Python utilizes proper spacing to distinguish blocks of code

Python uses a tab or 4 spaces to contain code in a code block. The example below prints "Hello, World!" 100 times.

for i in range(100):
    print("Hello, World!")

Data Types

string - str, ex: "This is a string"
integer - int, ex: 10
floating-point number - float, ex: 2.22
booleans - bool, ex: True
NonType - where None is the value

Python does not require you to specify the type when making variable declarations.

x = 100

100

Here x is assigned the value of 100.

Increment

x = 10
x = x + 1
print(x)

11

OR

x = 10
x += 1
print(x)

11

Arithmatic Operators

'**' Exponent

'%' Modulo/remainder

'//' Integer division/floor quotient

'/' Division

'*' Multiplication

'-' Subtraction

'+ 'Addition

Comparison Operators

'>' Greater Than

'<' Less Than

'>=' Greater than or equal to

'<=' Less than or equal to

'==' Equal

'!=' Not Equal

Logical Operators

and - True and True -> True

or - True or False -> True

not - not True -> False

Conditional Statements

Conditional statements can be constructed with keywords: if, elif & else

Given x = 3. The following conditional statment would yield

x is not 1 or 2

if x == 1:
    print("x is 1")
elif x == 2:
    print("x is 2")
else:
    print("x is not 1 or 2")

Functions

Use keyword def to define your function and then give your function a name along with parameters to pass in parenthesis

def functionName(x):
    return x + 1
    

functionName(2)

3

Built-in Functions

len() - returns the length of the object passed to it
str() - returns the object that gets passed to it as a string
int () - returns the object that gets passed to it as a integer
float() - returns the object that gets passed to it as a float
input(x) - prints x to the console; pauses program; takes in a string that is typed into console

Required and Optional Parameters

Python lets you set optional parameters in a function by giving the optional parameter a default value

def functionName(x, y = 2):
    return x + y

Here x is required and y is optional.

functionName(1, 4)

5

functionName(1)

3

Variable Scope

Global Variable - a variable with global scope that can be read or written to from anywhere in your program

Local Variable - a variable with local scope that can only be read or written to from within the function or class that it is in

Use keyword global when writing to a global variable from within a function

x = 2

def f():
    global x
    x += 1
    print(x)
    
f()

3

Error Handling

y = input("Give a value for y:")

try:
    x = int(y)
except ValueError:
    print("y must be a number")

Give a value for y:

Python

y must be a number

Python Exceptions:

SyntaxError: EOL while scanning string literal
ZeroDivisionError: division by zero
IndentationError: unexpected indent
ValueError: invalid literal for int() with base 10: 'exampleString'

Containers

Lists

a container that stores objects in a specific order
iterable
mutable

list = [] list = list()

use index such as list[1] to find object in the list at a specific index

use append() to add to a list

use pop() to remove last item from list

add 2 lists with the addition operator

use keyword in to check if an item is in a list

use keyword not paired with in to check if an item is not in a list

Tuples

a container that stores objects in a specific order
immutable
iterable

tuple = () tuple = tuple()

use index such as tuple[1] to find object in the tuple at a specific index

use keywords in and not just like lists

Dictionaries

a container that maps a key to a value

dictionary = {} dictionary = dict()

add a value using the following format:

dictionary["key1"] = "value1"
dictionary["key2"] = "value2"

{"key1":"value1", "key2":"value2"}

use keywords in and not just like lists

Containers can contain different containers.

Strings

strings are immutable
use an index to look up a character, indexes start at 0
a negative index can be used to look up items from right to left where [-1] gives the last character in a string

string = "Jenaba" string[0] = J string[2] = n string[-1] = a string[-2] = b
triple quotes can be used to surround a string that has multiple lines

string = """line 1 line 2 line 3 """
use '+' to add strings together

"string1" + "string2"

"string1string2"

use the asterisk to multiply strings

"string1" * 2

"string1string1"

Built-in Functions for Strings

.upper() - turns string to all upper case characters

.lower() - turns string to all lower case characters

.capitalize() - capitalizes the first letter of a sentence

.format(x) - will insert into the string to replace the empty {}

        "Hello, my name is {}".format("Noelle")

"Hello, my name is Noelle"

.split(x) - splits a string into a list at each x

"blue, pink, purple".split(", ")

["blue", "pink", "purple"]

.join() - used to add new characters between characters of a string; can also be used to turn a list of strings into a string with a new character in between

".".join("python")

"p.y.t.h.o.n"

listOfStrings = ["blue", "pink", "purple"]
newString = " ".join(listOfStrings)

"blue pink purple"

.strip() - strips string of excess white space

string = "      python    "
string.strip()

"python"

.replace(x, y) - when passed two arguments x & y, will replace x with y

string = "Hello World"
string.replace("l", "!")

"He!!o Wor!d"

.index(x) - returns the index of the first occurrence of x in a string

string = "Hello World"
string.index('o')

4

use the keyword in to check if a string contains another string
escape strings within strings by using the backslash (")

"I said "Hello World""
\n in a string creates a new line
use the syntax string[x:y] to slice a string from index x up to index y

string = "Hello World" string[0: 5]

"Hello"

slicing can be used on Lists as well

Loops

For-Loop

for-loops are used to iterate through an iterable (string, list, etc.)
```
 name = "Ted"
 for character in name:
      print(character)
```

T e d

you can also have an index variable in a for-loop by using enumerate(x):

tv = ["GOT", "Narcos", "Vice"] for i, show in enumerate(tv): new = tv[i] new = new.upper() tv[i] = new

print(tv)

['GOT', 'NARCOS', 'VICE']

or by using range(x,y):

for i in range(1, 11): print(i)

1

...

9 10

While-Loop

while-loops are used to run a block of code as long as a condition is true

x = 3 while x > 0: print(x) x -= 1 print("Happy New Year!")

3

2

1

'Happy New Year!'

Note: writing 'while True:' will make an infinite loop unless you use a break statement

#####Break

a break statement is used to terminate a loop

while True: a = input("Guess a number: ") if a == '87': break else print("That's not the right number!"

#####Continue

a continue statement stops the current iteration & moves onto the next one

for i in range(1, 6): if i == 3: continue print(i)

1

2

4

5

Nested Loops

you can combine loops by putting, or 'nesting', one loop inside another

list1 = [1, 2, 3] list2 = [4, 5, 6] added = [] for i in list1: for j in list2: added.append(i + j)

print(added)

[5, 6, 7, 6, 7, 8, 7, 8, 9]

Modules

another name for a python file with code in it
you can import a module using keyword import to use variables and functions from it

Built-in Modules:

math
random
statistics
keyword

import math

math.pow(2, 3)

8.0

Create your own module by:

create a python file
save the file
import the file into another file located in the same folder

Files

python has built-in functions made to manipulate file objects
first step is to open a file using the open() function
the write(x) function lets you write x to a file
the read() function lets you read from a file
you must close a file with the close() function

Open file

the open function takes a file and a file mode and returns a file object
Here are some modes that you can open a file in :
- "r" - opens a file for reading only
- "w" - opens a file for writing only; overwrites the file if it already exists; if it doesn't exist, creates a new file
- "w+" - opens a file for reading and writing; overwrites the file if it already exists; if it doesn't exist, creates a new file
st = open("st.text", "w") st.write("Hi from Python!") st.close()

Automatically close file

to automatically close a file without using the *close() function, open your file using a with-statement

with open("st.txt", "w") as f: f.write("Hi from Python!")
this will automatically close the file once the code inside the with-statement executes

CSV files

python has built-in module for working with CSV files
you can use the open() function to open a file and then use the writer() function from the csv module to turn the file object into a csv object
use the function writerow() to write to the csv object

import csv

with open("st.csv", "w", newline='') as f: w = csv.writer(f, delimiter=",") w.writerow(["one", "two", "three"]) w.writerow(["four", "five", "six"])

one,two,three

four,five,six

the csv module also has a function for reading a CSV file: reader()
the reader() function returns an iterable of the rows in the csv file

import csv

with open("st.csv", "r") as f: r = csv.reader(f, delimiter=",") for row in r: print(",".join(row))