File system

Path separators

You’ll need to use paths a lot because they are how you navigate the directory structure, and uniquely identify files.

The character that goes between the directory (and file) names in the path is called the path separator. Remember that it is different on the Windows operating system:

  • on Unix (includes Macs, and paths in URLs): /

  • on Windows: \

On the command line, separators convey critical information: spaces between commands, options, and arguments, and path separators in file paths. For this reason, be disciplined and sensible when you are naming files.

Path separators in your programs

When you are typing on a command line, will normally know if you’re working on a Unix or a Windows system, so you can choose the right separator. But if you write a program that uses file paths — and sooner or later you will — you can run into problems if it’s using the wrong path separators.

Suppose you write a Python program that tries to identify a data file using a path with / separating the directory names. That program may work fine on a Unix machine. But the same program, with no changes, may fail to find its data file when you (or someone else) run it on Windows.

The right way to handle this is to avoid using hard-coded / or \ characters in your paths, and delegate the problem to the operating system. Whatever programming language you are using, there will be a mechanism for dealing with this because it is a universal problem.

In Python

There’s more than one way of doing this, but Python 3.4+ has a pathlib module to solve this problem for you. The Path object will use the correct separators for whichever system the program is running on.

For example:

from pathlib import Path

data_path = Path("data", "raw", "ex-1.dat")

# data_path will contain:
# - "data/raw/ex-1.dat" on Unix/Mac
# - "data\raw\ex-1.dat" on Windows

Older Python programs use the os (operating system) module which contained similar features (and os.sep contained the correct separator character).

In Java

The Java library io.File contains File.separator (a string) and File.separatorChar (a character). They will be correct for whichever system your (compiled) program is running on.


public class ExampleFilePath {
    public static void main(String[] args) {
        System.out.println("data" + File.separator + "raw"
          + File.separator + "ex-1.dat"