1. Opening Files with open()

To work with a file in Python, you first need to open it. The open() function creates a connection between your program and a file on your computer.

# Open a file for reading
file = open("greeting.txt", "r")

# "r" is the default, so this also works:
file = open("greeting.txt")

# Always close the file when done
file.close()

Think of opening a file like opening a book. You need to open it before you can read it, and you should close it when you are done.

2. Reading Entire Contents with read()

The read() method reads the entire file as a single string:

Hello, World!
Welcome to Python.
Have a great day!

file = open("greeting.txt", "r")
content = file.read()
print(content)
file.close()

The read() method returns everything in the file as one big string, including the newline characters (\n) between lines.

3. Reading One Line with readline()

The readline() method reads one line at a time. Each call reads the next line:

file = open("greeting.txt", "r")

line1 = file.readline()
print("Line 1:", line1.strip())

line2 = file.readline()
print("Line 2:", line2.strip())

line3 = file.readline()
print("Line 3:", line3.strip())

file.close()

4. Reading All Lines as a List with readlines()

The readlines() method reads all lines and returns them as a list of strings:

file = open("greeting.txt", "r")
lines = file.readlines()
print(lines)
file.close()

Notice each line still has \n at the end. You can process the list to clean them up:

file = open("greeting.txt", "r")
lines = file.readlines()
file.close()

# Clean up each line
for line in lines:
    print(line.strip())

5. Iterating Through a File

The most Pythonic and memory-efficient way to read a file line by line is to iterate directly over the file object:

file = open("greeting.txt", "r")

for line in file:
    print(line.strip())

file.close()

This approach is best for large files because it only loads one line into memory at a time, rather than loading the entire file at once.

6. File Paths

When opening a file, you need to tell Python where the file is located. There are two types of paths:

# Relative path - relative to where your script is running
file = open("data.txt")              # Same folder
file = open("data/scores.txt")       # Subfolder called "data"
file = open("../notes.txt")           # Parent folder

# Absolute path - full path from the root
# Mac/Linux:
file = open("/Users/alice/data.txt")

# Windows (use raw string to avoid backslash issues):
file = open(r"C:\Users\alice\data.txt")