Python Principles

Python Programming Reference Sheet

Basics

Print a number print(123)
Print a string print("test")
Adding numbers print(1+2)
Variable assignment number = 123
Print a variable print(number)
Function call x = min(1, 2)
Comment # a comment

Types

Integer 42
String "a string"
List [1, 2, 3]
Tuple (1, 2, 3)
Boolean True

Useful functions

Write to the screen print("hi")
Calculate length len("test")
Minimum of numbers min(1, 2)
Maximum of numbers max(1, 2)
Cast to integer int("123")
Cast to string str(123)
Cast to boolean bool(1)
Range of numbers range(5, 10)

Other syntax

Return a value return 123
Indexing "test"[0]
Slicing "test"[1:3]
Continue to next loop iteration continue
Exit the loop break
List append numbers = numbers + [4]
List append (with method call) numbers.append(4)

Terminology

syntax the arrangement of letters and symbols in code
program a series of instructions for the computer
print write text to the screen
string a sequence of letters surrounded by quotes
variable a storage space for values
value examples: a string, an integer, a boolean
assignment using = to put a value into a variable
function a machine you put values into and values come out
call (a function) to run the code of the function
argument the input to a function call
parameter the input to a function definition
return value the value that is sent out of a function
conditional an instruction that's only run if a condition holds
loop a way to repeatedly run instructions
list a type of value that holds other values
tuple like a list, but cannot be changed
indexing extracting one element at a certain position
slicing extracting some elements in a row
dictionary a mapping from keys to values

Reminders

  • Strings and lists are indexed starting at 0, not 1
  • Print and return are not the same concept
  • The return keyword is only valid inside functions
  • Strings must be surrounded by quotes
  • You cannot put spaces in variable or function names
  • You cannot add strings and integers without casting
  • Consistent indentation matters
  • Use a colon when writing conditionals, function definitions, and loops
  • Descriptive variable names help you understand your code better

Conditionals

if x == 1:
	print("x is one")
else:
	print("x is not one") 

Defining functions

def my_func (param1, param2):
	result = param1 + param2
	return result 

Loops

for number in [1, 2, 3]:
	print(number)

x = 0
while x < 10:
	print(x)
	x = x + 1 

Dictionaries

numbers = {
	1: "one",
	2: "two"
}

print(numbers[1]) 

Comparisons

Equals ==
Not equals !=
Less than <
Less than or equal <=
Greater than >

Useful methods

String to lowercase "xx".lower()
String to uppercase "xx".upper()
Split string by spaces "a b c".split(" ")
Remove whitespace around string " a string ".strip()
Combine strings into one string " ".join(["a", "b"])
String starts with "xx".startswith("x")
String ends with "xx".endswith("x")
List count [1, 2].count(2)
List remove [1, 2].remove(2)
Dictionary keys {1: 2}.keys()
Dictionary values {1: 2}.values()
Dictionary key/value pairs {1: 2}.items()

Other neat bonus stuff

Zip lists zip([1, 2], ["one", "two"])
Set my_set = {1, 2, 3}
Set intersection {1, 2} & {2, 3}
Set union {1, 2} | {2, 3}
Index of list element [1, 2, 3].index(2)
Sort a list numbers.sort()
Reverse a list numbers.reverse()
Sum of list sum([1, 2, 3])
Numbering of list elements for i, item in enumerate(items):
Read a file line by line for line in open("file.txt"):
Read file contents contents = open("file.txt").read()
Random number between 1 and 10 import random; x = random.randint(1, 10)
List comprehensions [x+1 for x in numbers]
Check if any condition holds any([True, False])
Check if all conditions hold all([True, False])

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