Paths and Where Your Program Runs

"File not found" is usually a lie of omission. The file exists; it just is not where the program is looking. The reason is simple once you know it: a file name on its own is looked up in the folder the terminal is in, not the folder the script is in. This page explains that, shows how to build paths that work on every system, and gives you a line that finds a file next to your script wherever the terminal happens to be.

Think of directions that say "the second door on the left". They work only if everyone agrees where you are standing. A full street address works from anywhere. File paths come in the same two kinds.

Relative and Absolute

A path says where a file is. An absolute path starts from the top of the disk, such as /Users/you/python-course/notes.txt on a Mac or C:\Users\you\python-course\notes.txt on Windows. It means the same thing from anywhere. A relative path, such as plain notes.txt, is looked up starting from the current folder, the one you met with pwd in Chapter 1.

The Current Folder Is the Terminal's

Take the simplest possible reading script, read.py, saved in python-course next to notes.txt.

with open("notes.txt", encoding="utf-8") as f:
    print(f.read())

When you run python3 read.py from inside python-course, the current folder is python-course, and notes.txt is found there. Run the same script from your home folder, as python3 python-course/read.py, and the current folder is your home folder. Python looks for notes.txt there, does not find it, and raises FileNotFoundError, even though the file is sitting right next to the script.

you@mac ~ % python3 python-course/read.py
Traceback (most recent call last):
  File "/Users/you/python-course/read.py", line 1, in <module>
    with open("notes.txt", encoding="utf-8") as f:
         ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: 'notes.txt'

So the first question for any file-not-found error is: which folder was the terminal in? pwd answers it.

pathlib: Paths as Objects

Writing paths as plain strings means worrying about slashes: forward on a Mac, backward on Windows. The pathlib module removes that worry. A Path object represents a path, and the forward-slash operator joins parts together, producing the right separator for whichever system the program runs on. Paths also answer questions about themselves.

>>> from pathlib import Path
>>> p = Path("data") / "notes.txt"
>>> p
PosixPath('data/notes.txt')
>>> p.name
'notes.txt'
>>> p.suffix
'.txt'
>>> p.exists()
False

On a Mac or Linux the object prints as a PosixPath; on Windows it would be a WindowsPath written with a backslash. Your code is the same on both. The last line checks whether the file exists, without opening it, which is a good way to give a friendly message instead of a traceback.

Finding a File Next to the Script

For a file that lives alongside your script, there is a line that works wherever the terminal is. Inside any script, the name __file__ holds the script's own path. Python programmers read the two underscores on each side as "dunder", short for double underscore, so this one is said "dunder file". Path(__file__).parent is the folder containing the script, and joining the file name onto it gives an absolute path that does not depend on the current folder at all.

from pathlib import Path

notes = Path(__file__).parent / "notes.txt"
with open(notes, encoding="utf-8") as f:
    print(f.read())

This one line is worth copying now and understanding fully after Chapter 9, where modules and the double-underscore names are explained. It is the standard way scripts find the files that ship with them.