string.rstrip() is removing extra characters

Question

Why does this statement remove the 'E' in 'PIPELINE':

In: 'PIPELINE_DEV'.rstrip('_DEV')
Out: 'PIPELIN'

But this statement does not remove the 'S':

In: 'PIPELINES_DEV'.rstrip('_DEV')
Out: 'PIPELINES'

This statement removes all of the E's at the end:

In: 'PIPELINEEEEEEEE_DEV'.rstrip('_DEV')
Out: 'PIPELIN'

When I turn the rstrip into 2 separate statements, it works fine:

In: 'PIPELINE_DEV'.rstrip('DEV').rstrip('_')
Out: 'PIPELINE'

Answer 1

rstrip removes any trailing instances of the characters you supply from the string you apply it on until it finds something that doesn't match. This is stated in its documentation:

The chars argument is not a suffix; rather, all combinations of its values are stripped

Using rstrip('_DEV') it will remove _DEV from the string and then all Es (or 'D's or 'V's or '_'s) since those fall in the character set you've given (and no other character that isn't in that set has been found).

When you use .rstrip('DEV').rstrip('_') the first call to rstrip strips off 'DEV' and then stops because '_' isn't in the character set 'DEV'. '_' is then removed when the second call to rstrip is made.

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Note that in Python >= 3.9, str.removesuffix was added in order to address this common misconception. Using removesuffix, you can supply a suffix string that is removed, if present, as a suffix of the string it is applied on.

Your example:

'PIPELINE_DEV'.removesuffix('_DEV')

would only remove the '_DEV' suffix.