Python Files

File Modes

Opening & Closing a File

Manual open/close

file = open("file.txt", "r")
# ... file operations ...
file.close()   # always close — forgetting this can cause issues

⚠️ If an exception occurs before file.close(), the file stays open.

Preferred: with statement (context manager) ✅

with open("file.txt", "r") as file:
    # ... file operations ...
# file is automatically closed when the block ends — even if an exception occurs

File Paths

• If the file is in the same directory as where you run the script, use just the filename: "file.txt"
• If not, use the relative path from where the script is executed: "subfolder/file.txt"
• Or use the absolute path: "C:/Users/Tim/documents/file.txt"

Python looks for files relative to the execution directory (where you run the script from), not necessarily where the .py file lives.

Reading Files

file.read() — entire file as one string

with open("file.txt", "r") as file:
    content = file.read()
    print(content)

file.readlines() — list of lines (each includes \n)

with open("file.txt", "r") as file:
    lines = file.readlines()
    print(lines)   # ['Hello world\n', 'Add line\n', '\n', ...]

file.read(n) — read exactly n characters

with open("file.txt", "r") as file:
    print(file.read(5))   # first 5 characters only

Guide: \n (newline) counts as 1 character.

Handling \n (Newline Characters)

Every time you press Enter in a file, a \n is inserted. These come along when you read lines.

Strip trailing whitespace/newlines

line = "Hello world\n"
print(line.strip())   # "Hello world" — removes leading/trailing whitespace and \n

Remove all newlines from a string

line = line.replace("\n", "")

Iterating Over a File

Loop line by line without loading the whole file into memory:

with open("file.txt", "r") as file:
    for line in file:
        print(line, end="")   # end="" prevents double newlines

Read only the first N lines

with open("file.txt", "r") as file:
    for i, line in enumerate(file):
        if i == 2:   # 0-indexed: stops after 3rd line
            break
        print(line, end="")

Writing Files ("w" mode)

with open("file2.txt", "w") as file:
    file.write("Hello world\n")
    file.write("This is line 2\n")
    file.write("This is line 3\n")

• Creates the file if it doesn't exist
• Completely overwrites the file if it already exists ⚠️
• file.write() does not add newlines automatically — you must add \n yourself
• Only accepts strings — convert numbers with str() before writing

Appending to Files ("a" mode)

with open("file.txt", "a") as file:
    file.write("\nNew line added")

• Places cursor at the end of the file
• Adds to the file without erasing existing content
• Each run keeps adding more — careful with infinite growth

Read + Write ("r+" mode)

Opens for both reading and writing. Cursor starts at the beginning.

Common use case — read a value, update it

with open("score.txt", "r+") as file:
    score = file.read()           # read current value, cursor moves to end
    new_score = int(score) + 1
    file.seek(0)                  # move cursor back to beginning!
    file.write(str(new_score))    # overwrite with new value

⚠️ After reading, the cursor is at the end of the file. If you write without calling file.seek(0) first, you'll append instead of overwrite.

file.seek(position)

Moves the file cursor to a specific position:

Most commonly used: seek(0) to reset to the beginning after reading.

Key Takeaways & Recap