Relative & Absolute Paths

1A filename is not an address

“Asha” is not an address. There are lots of Ashas. “Asha, Flat 3, Nehru Road, Pune” is an address, because following it can only ever land you in one place.

Files work the same way. Folders sit inside folders, all the way up to the top of the disk, and a path is the list of folders you walk through to reach a file — written out with a separator between each step.

2The absolute path: start at the top

You can tell one by looking at how it starts. If it begins with C:\ or with /, it is absolute. That first character is the promise: begin at the top, do not guess.

3The bit nobody tells you: where your program is standing

Before relative paths can make any sense, you need one idea that is completely invisible on screen. When a program runs, it is always running inside some folder. That folder is called the current working directory — the CWD, or just “where you are”.

Normally it is the folder your .py file was saved in. You can always ask:

import os

print('Python is running in this folder:')
print(os.getcwd())

Python is running in this folder:
/home/admin/school/programs

4The relative path: start from where you are

When you write marks.csv with nothing in front of it, that is already a relative path. It means “a file called marks.csv, in whichever folder I am in”. Two special names let you move around:

5The same file, both ways

Below is a small set of folders. Pick a file, then change the folder the program is running in and watch what happens: the absolute path does not move at all, and the relative path rewrites itself every time.

6Watching Python resolve them

The folders in the widget are real ones, and this program was run inside programs. os.path.exists() answers “is there a file at this path?” and os.path.abspath() shows the full address a relative path stands for:

import os

print('marks.csv              ->', os.path.exists('marks.csv'))
print('syllabus.txt           ->', os.path.exists('syllabus.txt'))
print('../notes/syllabus.txt  ->', os.path.exists('../notes/syllabus.txt'))

print()
print('marks.csv really means:')
print(os.path.abspath('marks.csv'))

print()
print('../notes/syllabus.txt really means:')
print(os.path.abspath('../notes/syllabus.txt'))

marks.csv              -> True
syllabus.txt           -> False
../notes/syllabus.txt  -> True

marks.csv really means:
/home/admin/school/programs/marks.csv

../notes/syllabus.txt really means:
/home/admin/school/notes/syllabus.txt

Line two is the one to remember. “File not found” almost never means the file is missing. It means Python looked in the folder it was standing in and the file was somewhere else.

7The backslash trap

Windows separates folders with \. Python uses \ inside a string to start an escape sequence — \n for a newline, \t for a tab. Put a Windows path in quotes and those two facts collide.

path = "C:\new\table.txt"
print(path)

C:
ew	able.txt

Sometimes you are luckier and it does not run at all, which is easier to deal with because at least you find out:

path = "C:\Users\new\table.txt"
print(path)

  File "worse_path.py", line 1
    path = "C:\Users\new\table.txt"
           ^^^^^^^^^^^^^^^^^^^^^^^^
SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 2-3: truncated \UXXXXXXXX escape

\U starts a Unicode escape, and sers\new… is not one, so the file does not even get past the check-and-translate step. There are three ways to write a Windows path safely, and all three work:

doubled = "C:\\school\\marks.csv"
raw     = r"C:\school\marks.csv"
forward = "C:/school/marks.csv"

print(doubled)
print(raw)
print(forward)

8Which should you write?

9Recap