Fix dehumanize() silently ignoring negative time values

[amanv0007]

Fixes #1278

Problem

dehumanize("in -1 hours") silently returns the same result as dehumanize("in 1 hours") - it drops the minus sign without any warning.

I ran into this while building a scheduling tool that accepts relative time strings from users. A typo like "in -2 days" would silently schedule something 2 days ahead instead of raising an error, which took me a while to track down.

Root cause

The number-extraction regex in dehumanize() uses \d+ to match the numeric value from the input. Since \d only matches [0-9], the minus sign in -1 is never captured - the regex matches 1 and moves on. The sign information is completely lost.

Before this fix

>>> now = arrow.now()
>>> r1 = now.dehumanize("in 1 hours")
>>> r2 = now.dehumanize("in -1 hours")
>>> (r1 - now).total_seconds()
3600.0
>>> (r2 - now).total_seconds()
3600.0  # <- wrong, same as positive!

After this fix

>>> now.dehumanize("in -1 hours")
ValueError: Negative time values are not supported in dehumanize
(got 'in -1 hours'). Use a positive value with 'ago' or 'in' to
indicate direction.

Changes

• arrow/arrow.py: Added an early check in dehumanize() that detects negative numbers (-\d pattern) in the input string and raises a ValueError with a clear message, before the regex-based parsing begins.
• • tests/test_arrow.py: Added test_negative_values to verify that inputs like "in -1 hours", "in -2 days", "-3 minutes ago", and "in -30 seconds" all raise ValueError.
    All existing dehumanize tests continue to pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (a345b27).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1279   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2315      2317    +2     
  Branches       358       359    +1     
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+ Hits          2315      2317    +2     

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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds an early-rejection check in Arrow.dehumanize so that inputs containing negative numbers (e.g. "in -1 hours") raise ValueError instead of silently producing a wrongly-signed result (issue #1278).

Changes:

• Add a regex check at the start of dehumanize that raises ValueError when the input contains a negative number.
• Add a unit test verifying the new behavior across a few representative inputs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
arrow/arrow.py	Adds the negative-number guard with a descriptive ValueError in dehumanize.
tests/test_arrow.py	Adds test_negative_values covering several negative-number inputs.

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[amanv0007]

Thank you for the excellent suggestion! I have refined the regex check to r"(?:^|\s)-\d" so it is anchored to start-of-string or whitespace and won't trigger false positives on hyphen-joined strings in localized non-English timeframes. Pushed the update!

[amanv0007]

Done! I've added a non-negative test ensuring "in 1 hours" and "2 days ago" succeed, and verified that hyphenated strings without preceding spaces (like "some-2nd-hour") bypass the negative check as expected. Pushed the update!