fix: escape backslashes in set_key to preserve round-trip

[gaoflow]

Bug

set_key() writes values wrapped in single quotes, and the parser decodes \\ as a literal backslash via decode_escapes(). But set_key() only escaped single-quote characters (\'), not backslashes themselves. This meant any value containing two or more consecutive backslashes lost one during round-trip:

Also reproduces for Windows paths (C:\Users\...) and any value with \\ sequences.

Root Cause

The parser's _single_quote_escapes regex matches both \' and \\, decoding them via codecs.decode(..., 'unicode-escape'). When set_key writes a value containing literal \\, the parser interprets it as an escaped backslash and decodes it to \, losing one backslash on each consecutive pair.

Fix

Escape backslashes in addition to single-quotes in set_key(), before wrapping in single quotes. The order is important: backslashes must be escaped first (\\ -> \\\\), then single-quotes (' -> \').

Diff

• src/dotenv/main.py: +2/-0 (escape backslashes before single-quotes)
• tests/test_main.py: +23 (parametrized round-trip regression test covering 0-3 backslashes, Windows paths, and mixed quotes+backslashes)

[theskumar]

Hey @gaoflow - thank you for this, and for the clear root-cause writeup, it made the bug easy to follow. I'm consolidating the backslash fix for #661 into #680, which builds on the same idea and also fixes the parser side (a trailing backslash used to swallow the next line). Closing this in its favor - nothing wrong with your patch, just landing one. Thanks again!