Why are backslashes in strings doubled up in my JSON output?

Question

I want to be able to use backslashes in string literals, for example: C:\file.txt

I was expecting to be able to escape the backslash with "\\", but for some weird reason that produces two backslashes instead of one. Why?

MVCE:

with open("temp.csv", 'w', encoding='utf-8') as f:
    d = { "filepath" : "C:\\bahä.txt" }
    s = json.dumps(d, ensure_ascii=False)
    f.write(s)

Expected behavior: file content should be C:\bahä.txt

Actual behavior: file content is C:\\bahä.txt

Answer 1

In JSON you need to escape the backslash. That is why when you dump your string it keeps the backslash escaped. if you have only one backslash, e.g. "C:\bahä.txt" then \b is backspace (i.e. if it wasn't b after the backslash to produce valid escape sequence it would be invalid json). you can test and see that

import json
with open("temp.json", 'w', encoding='utf-8') as f:
    d = {"filepath":"\ "}
    s = json.dumps(d, ensure_ascii=False)
    f.write(s)

or

import json
with open("temp.json", 'w', encoding='utf-8') as f:
    d = { "filepath" : r"\b" }
    s = json.dumps(d, ensure_ascii=False)
    f.write(s)

both will again produce escaped backslash (i.e. \\) in the resulting json.

https://stackoverflow.com/a/19176131/4046632

As a side note - you are writing json, use the proper extension - .json, not .csv

Answer 2

The problem here is that you are serializing to JSON.

In Json some special characters are serialized with a \ prepended: https://stackoverflow.com/a/19176131/551045

So a \ is always \\ in proper serialized JSON data.

If you change your code to:

with open("temp.csv", 'w', encoding='utf-8') as f:
    d = "C:\\bah.txt"
    f.write(d)

You'll see that your file will only contain one slash.

BTW that's why we always ask for MVCE even if the problem seems "trivial".