+= is not equal to + when merging JUnitXml variables

[miguelflor]

Description of the problem

I can't figure it out how to merge two JUnitXml variables.
And I have different outputs for += and + operators.

Code

Example 1:

JUnitXml.__str__ = lambda self: (
    f"{(self.tests or 0) - (self.failures or 0)}/{self.tests}"
)

logger.debug(
    "Merging: {} with {} = {}",
    junits,
    junits_merge,
    junits + junits_merge,
)
junits_merge += junits_merge
logger.info("Junit Output: {}", junits_merge)

output:

2026-05-12 11:44:03.112 | DEBUG    | __main__:main:149 - Merging: 177/262 with 40/40 = 217/302
2026-05-12 11:44:03.114 | INFO     | __main__:main:157 - Junit Output: 40/40

If I added this line after the += operation:

junits_merge.update_statistics()

The result is the same as Example 2.

Example 2:

JUnitXml.__str__ = lambda self: (
   f"{(self.tests or 0) - (self.failures or 0)}/{self.tests}"
)

logger.debug(
   "Merging: {} with {} = {}",
   junits,
   junits_merge,
   junits + junits_merge,
)
junits_merge = junits +  junits_merge
logger.info("Junit Output: {}", junits_merge)

output:

2026-05-12 12:45:45.671 | DEBUG    | __main__:main:150 - Merging: 177/262 with 40/40 = 217/302
2026-05-12 12:45:45.673 | INFO     | __main__:main:157 - Junit Output: 0/0

Versions

• Python: 3.12.3
• junitparser: 5.0.0

Additional notes

If you think there is not enough details feel free to ask for more!
And note that I am considering that the "problem" can be on my side, I am just asking directly to the pros because I am running out of options.
Thank you!

[miguelflor]

I corrected it. But this does not look like what the expected behavior should be.
I needed to remove the junit + junits_merge from the logger.
new code:

JUnitXml.__str__ = lambda self: (
   f"{(self.tests or 0) - (self.failures or 0)}/{self.tests}"
)

logger.debug(
   "Merging: {} with {}",
   junits,
   junits_merge,
)
junits_merge = junits +  junits_merge
logger.info("Junit Output: {}", junits_merge)

outputs:

2026-05-12 16:23:11.692 | DEBUG    | __main__:main:153 - Merging: 177/262 with 40/40
2026-05-12 16:23:11.693 | INFO     | __main__:main:160 - Junit Output: 217/302