Python Programming Basics
Hello world
print("Hello, world!")
Comments
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main function
Python does not technically require a main function like C. You can simply write your code at the top level:
print("Hello, world!")
But it’s a good practice to put your code inside a function:
def main():
print("Hello, world!")
if __name__ == "__main__":
main()
Packages
You can import packages in Python using the import keyword. For example, to import the math package:
import math
print(math.sqrt(16))
You can also import specific functions from a package:
from math import sqrt
print(sqrt(16))
Indentation
Python uses indentation to define blocks of code. For example, in the following code snippet, the print("Hello") statement is indented to indicate that it is part of the if block:
x = 5
if x > 0:
print("Hello")
Variables
Declaring variables
Unlike C, you don’t need to specify the type of a variable in Python. You can simply assign a value to a variable:
x = 5
y = "Hello"
z = 3.14
Data Types
Python has several built-in data types, including:
int: Integerfloat: Floating-point numberstr: Stringbool: Boolean
You can check the type of a variable using the type() function:
x = 5
print(type(x)) # <class 'int'>
y = "Hello"
print(type(y)) # <class 'str'>
Type Conversion
You can convert variables from one type to another using the built-in functions int(), float(), str(), and bool():
x = 5
y = str(x)
print(y) # "5"
Operators
Arithmetic Operators
Python supports the following arithmetic operators:
+: Addition-: Subtraction*: Multiplication/: Division//: Floor division%: Modulus**: Exponentiation
Relational Operators
Python supports the following comparison operators:
==: Equal to!=: Not equal to<: Less than>: Greater than<=: Less than or equal to>=: Greater than or equal tois: Identityis not: Negated identityin: Membershipnot in: Negated membership
Logical Operators
Logical operators in Python include:
and: Logical ANDor: Logical ORnot: Logical NOT
Assignment Operators
Python supports the following assignment operators:
=: Assign+=: Add and assign-=: Subtract and assign*=: Multiply and assign/=: Divide and assign%=: Modulus and assign**=: Exponentiate and assign//=: Floor divide and assign
Input/Output and Printing
Input
You can get user input using the input() function:
name = input("Enter your name: ")
print(f"Hello, {name}!")
print
You can print output using the print() function:
print("Hello, world!")
You can also use f-strings to format output:
name = "Alice"
print(f"Hello, {name}!")
format
You can also use the format() method to format strings:
name = "Alice"
print("Hello, {}!".format(name))
sep and end
You can specify the separator and end character for the print() function:
print("Hello", "world", sep=", ", end="!")
# Hello, world!
Loops and Conditionals
If Statements
You can use an if statement to conditionally execute code:
x = 5
if x > 0:
print("Positive")
elif x < 0:
print("Negative")
else:
print("Zero")
For Loops
You can use a for loop to iterate over a sequence of elements:
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
print(fruit)
You can also use the range() function to generate a sequence of numbers:
for i in range(5):
print(i)
While Loops
You can use a while loop to repeatedly execute code as long as a condition is true:
x = 0
while x < 5:
print(x)
x += 1
Break and Continue
You can use the break statement to exit a loop prematurely:
for i in range(10):
if i == 5:
break
print(i)
You can use the continue statement to skip the rest of the loop and continue with the next iteration:
for i in range(5):
if i == 2:
continue
print(i)
Functions
Defining Functions
You can define functions in Python using the def keyword:
def greet(name):
print(f"Hello, {name}!")
Calling Functions
You can call a function by using its name followed by parentheses:
greet("Alice")
Return Values
Functions can return values using the return keyword:
def add(x, y):
return x + y
result = add(3, 5)
print(result) # 8
Default Arguments
You can specify default values for function arguments:
def greet(name="World"):
print(f"Hello, {name}!")
greet() # Hello, World!
greet("Alice") # Hello, Alice!
Variable Arguments
You can pass a variable number of arguments to a function using the *args syntax:
def add(*args):
total = 0
for num in args:
total += num
return total
result = add(1, 2, 3, 4, 5)
print(result) # 15
Keyword Arguments
You can pass keyword arguments to a function using the **kwargs syntax:
def greet(**kwargs):
print(f"Hello, {kwargs['name']}!")
greet(name="Alice") # Hello, Alice!
Lists
Lists in Python are ordered collections of elements that can be of different types. They are essentially arrays in other programming languages like C.
Creating Lists
You can create lists in Python using square brackets:
fruits = ["apple", "banana", "cherry"]
Accessing Elements
You can access elements of a list using their index:
print(fruits[0]) # apple
Length of a List
You can get the length of a list using the len() function:
print(len(fruits)) # 3
Slicing / Substrings
You can slice a list to get a substring of its elements:
print(fruits[1:3]) # ['banana', 'cherry']
Modifying Lists
You can modify elements of a list:
fruits[1] = "orange"
Adding Elements
You can add elements to a list using the append() method:
fruits.append("kiwi")
Removing Elements
You can remove elements from a list using the remove() method:
fruits.remove("banana")
List Comprehensions
You can use list comprehensions to create lists in a concise way:
squares = [x**2 for x in range(5)]
Using Strings like Lists
Strings in Python can be treated like lists of characters. But unlike lists, strings are immutable, meaning you cannot modify them.
s = "Hello, world!"
# Accessing elements
print(s[0]) # H
# Length of a string
print(len(s)) # 13
# Slicing
print(s[7:12]) # world
# Modifying elements
# s[7] = "W" # Error: strings are immutable
# Adding elements
# s.append("!") # Error: strings are immutable
# Removing elements
# s.remove(",") # Error: strings are immutable
# List comprehensions
chars = [c for c in s]
print(chars)
# Joining elements
s2 = "".join(chars)
print(s2)
# Splitting strings
words = s.split(", ")
print(words)
Dictionaries
Dictionaries in Python are collections of key-value pairs, where each key is associated with a value. They are similar to hash tables in other programming languages like C.
Creating Dictionaries
You can create dictionaries in Python using curly braces:
person = {"name": "Alice", "age": 30}
Accessing Elements
You can access elements of a dictionary using their keys:
print(person["name"]) # Alice
Modifying Elements
You can modify elements of a dictionary:
person["age"] = 25
Adding Elements
You can add elements to a dictionary by assigning a value to a new key:
person["city"] = "New York"
Removing Elements
You can remove elements from a dictionary using the pop() method:
person.pop("age")
Dictionary Comprehensions
You can use dictionary comprehensions to create dictionaries in a concise way:
squares = {x: x**2 for x in range(5)}
File I/O
Reading Files
You can read the contents of a file using the open() function:
with open("file.txt", "r") as file:
contents = file.read()
print(contents)
Writing Files
You can write to a file using the open() function with the mode set to "w":
with open("file.txt", "w") as file:
file.write("Hello, world!")
Appending to Files
You can append to a file using the mode set to "a":
with open("file.txt", "a") as file:
file.write("Hello, world!")
Reading and Writing CSV Files
You can read and write CSV files using the csv module:
import csv
with open("data.csv", "r") as file:
reader = csv.reader(file)
for row in reader:
print(row)
with open("data.csv", "w") as file:
writer = csv.writer(file)
writer.writerow(["Name", "Age"])
writer.writerow(["Alice", 30])