Feature: make JSON indentation configurable when merging COCO files

[philenius]

Thank you for providing this great library. I had the issue that merging two COCO files (each 2.8 MB in size) resulted in a 19 MB large COCO JSON file (too large in my opinion). The huge difference in size was caused by the JSON indentation which is set to 2 spaces:

pyodi/pyodi/apps/coco/coco_merge.py

Lines 90 to 91 in d48b39c

	with open(output_file, "w") as f:
	json.dump(output, f, indent=2)

Previously:

Merging two COCO files with fixed indentation of 2 spaces:

from pyodi.apps import coco
coco.coco_merge('coco_annotations1.json', 'coco_annotations2.json', 'output.json')

resulted in:

-rw-rw-r--  1 phil phil 2,8M Dez 13 18:59 coco_annotations1.json
-rw-rw-r--  1 phil phil 2,8M Dez 13 18:59 coco_annotations2.json
-rw-rw-r--  1 phil phil  19M Dez 13 19:01 output.json

With my changes:

Merging the same two COCO files with indentation disabled by setting json_indentation=None:

from pyodi.apps import coco
coco.coco_merge('coco_annotations1.json', 'coco_annotations2.json', 'output.json', json_indentation=None)

results in:

-rw-rw-r--  1 phil phil 2,8M Dez 13 18:59 coco_annotations1.json
-rw-rw-r--  1 phil phil 2,8M Dez 13 18:59 coco_annotations2.json
-rw-rw-r--  1 phil phil 5,6M Dez 13 19:01 output.json

[daavoo]

Thanks for the contribution!

I would suggest adding a simple check (in https://github.com/Gradiant/pyodi/blob/master/tests/apps/test_coco_merge.py) ensuring that indent is passed to json.dump.

[philenius]

@daavoo thank you for your feedback. I applied all of your suggestions.

As requested, I have added a unit test where I patch the call to json.dump() and assert that it was called with indent=None and indent=2. I hope I have understood you correctly.