Files and Paths¶

Try This First: Before reading, try this in Python: open('test.txt', 'w').write('hello') then print(open('test.txt').read()). You just wrote to a file and read it back.

Learn Your Way¶

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Programs read data from files and write results to files. Understanding how to work with files and their locations (paths) is essential.

Visualize It¶

See how Python reads a file and processes it line by line: Open in Python Tutor

Reading a file¶

The simplest way:

contents = open("data.txt").read()
print(contents)

The better way (automatically closes the file when done):

with open("data.txt") as f:
    contents = f.read()
    print(contents)

Reading line by line¶

with open("data.txt") as f:
    for line in f:
        line = line.strip()  # Remove the newline character
        print(line)

Writing to a file¶

with open("output.txt", "w") as f:
    f.write("Hello, world!\n")
    f.write("Second line.\n")

"w" means write (creates new file or overwrites existing)
"a" means append (adds to end of existing file)
"r" means read (default, what open() uses without a mode)

What is a path?¶

A path is the address of a file on your computer: - Windows: C:\Users\alice\projects\data.txt - Mac/Linux: /Users/alice/projects/data.txt

Relative vs absolute paths¶

Absolute: Full address from the root — C:\Users\alice\projects\data.txt
Relative: Address from where you are now — data.txt or ../other_folder/file.txt

.. means "go up one folder." So ../data.txt means "go up one folder, then find data.txt."

Using pathlib (the modern way)¶

from pathlib import Path

# Create a path
data_file = Path("data/sample.txt")

# Check if it exists
if data_file.exists():
    contents = data_file.read_text()

# Get parts of the path
data_file.name       # "sample.txt"
data_file.stem       # "sample"
data_file.suffix     # ".txt"
data_file.parent     # Path("data")

You will use pathlib starting in Level 0. For Level 00, plain open() is fine.

Common mistakes¶

File not found:

open("data.txt")  # FileNotFoundError if data.txt is not in your current folder

Fix: make sure you are in the right directory (cd to the project folder first).

Forgetting to strip newlines:

for line in open("data.txt"):
    print(line)  # Double-spaced output because each line has \n
    # Fix: print(line.strip())

Using backslashes on Windows:

# Wrong (backslash is an escape character)
path = "C:\Users\alice\new_file.txt"  # \n becomes a newline!

# Right
path = "C:\\Users\\alice\\new_file.txt"  # Escaped backslashes
path = r"C:\Users\alice\new_file.txt"    # Raw string (r prefix)
path = Path("C:/Users/alice/new_file.txt")  # Forward slashes work too

Practice¶

Quick check: Take the quiz

Review: Flashcard decks Practice reps: Coding challenges

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