Sub logging support to fancylog

[HARSHDIPSAHA]

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Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?
Introduce sublogging.
What does this PR do?
This pr adds a sublog feature whose parent will a main logger to consistently and easy manage and analyse loggings in heirarchial form.
adds SubLog and the sub_log context manager to fancylog, addressing both parts of the feature request:

• Separate log files per pipeline step, each linked from the main log
• Subprocess capture — stdout/stderr from third-party tools (e.g. NiftyReg, elastix) are routed into their own sub-log

References

#66

How has this PR been tested?

using pytest on added tests.

Is this a breaking change?

No

Does this PR require an update to the documentation?

Maybe

Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit

[HARSHDIPSAHA]

example use

import fancylog
from fancylog.sublog import sub_log

fancylog.start_logging(output_dir, fancylog, logger_name="pipeline")

with sub_log("registration", output_dir, parent_logger_name="pipeline") as sl:
    sl.logger.info("Starting atlas registration")
    sl.run_subprocess(["niftyreg", "-ref", "atlas.nii", "-flo", "brain.nii"])

Example output

fancylog_<timestamp>.log (main log — references only, no detail noise)

2026-02-28 16:17:54 PM - INFO - Starting sub-log 'preprocessing', see logs\preprocessing.log for details
2026-02-28 16:17:54 PM - INFO - Sub-log 'preprocessing' finished, log saved to logs\preprocessing.log
2026-02-28 16:17:54 PM - INFO - Starting sub-log 'registration', see logs\registration.log for details
2026-02-28 16:17:54 PM - INFO - Sub-log 'registration' finished, log saved to logs\registration.log
2026-02-28 16:17:54 PM - INFO - Starting sub-log 'external_tool', see logs\external_tool.log for details
2026-02-28 16:17:54 PM - INFO - Sub-log 'external_tool' finished, log saved to logs\external_tool.log

[codecov]

Codecov Report

❌ Patch coverage is 88.40580% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.35%. Comparing base (c987796) to head (7d880ff).

Files with missing lines	Patch %	Lines
fancylog/sublog.py	88.23%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #93      +/-   ##
==========================================
+ Coverage   77.72%   80.35%   +2.63%     
==========================================
  Files           3        4       +1     
  Lines         211      280      +69     
==========================================
+ Hits          164      225      +61     
- Misses         47       55       +8     

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