How to replace a string conditionally in Python?

Question

I need to look for a substring in a string and if I don't find it, perform an action. So I do this, and it works fine:

if 'AAA' not in myString: 
   print "AAA is not in myString"

What I need, however, is to avoid cases where AAA is a part of XAAA. In other words, I don't want my script to notice any presence of "XAAA" when it does if 'AAA' not in myString. So how can I modify the above if-clause to the following:

if 'AAA' not in myString except for cases where 'AAA' is a part of 'XAAA':
    print "AAA is not in myString"

I'd suggest renaming this question. Something like "How to replace a string conditionally in Python" — Adam Hughes
– Adam Hughes, Commented Jul 31, 2017 at 20:09

Gribouillis · Accepted Answer · 2017-07-31 20:11:57Z

8

I like re module solutions

if not re.search(r'(?<!X)AAA', myString):
    ...

answered Jul 31, 2017 at 20:11

Gribouillis

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Comments

Chris · Accepted Answer · 2017-07-31 20:09:28Z

5

I don't want my script to notice any presence of "XAAA" when it does if 'AAA' not in myString.

You can first remove any occurrences of the sub-string XAAA from myString, and then test if AAA is not a sub-string of myString:

if 'AAA' not in myString.replace('XAAA', ''):
    # do action

answered Jul 31, 2017 at 20:09

Chris

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1 Comment

Gribouillis Over a year ago

I'm not sure this works with XAAAA or AAXAAAA. The question was unclear about this case.

Anthony Pham · Accepted Answer · 2017-07-31 20:19:32Z

3

There are two ways.

You can remove XAAA from the string then compare:

if 'AAA' not in myString.replace('XAAA', ''):
    # do something

Or you can check for it first then compare:

if 'XAAA' not in myString:
    if 'AAA' not in myString:
        # do something

Two-liner:

if 'XAAA' not in myString and 'AAA' not in myString:
    # do something

edited Jul 31, 2017 at 20:19

answered Jul 31, 2017 at 20:13

Anthony Pham

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Comments

Mike Robinson · Accepted Answer · 2017-07-31 20:27:47Z

0

Regular expressions might serve you extremely-well here.

For instance, you could match the string against the so-called "regex," /[^X]?AAA/.

This regex tests for the existence of any string AAA which is not preceded by the character X, anywhere in the string.

(The ? is needed to support the possibility that the AAA occurs at the beginning of the string, where no X could possibly precede it.)

Every programming language supports regular expressions ... "and this is why."

(FYI: "regular expressions are an immensely-deep pool of water, that it is very well worth exploring." In practical application, it produces many "practical applications," such as this one, even if all that you ever do is to skim the surface of it and very-happily stay there.)

edited Jul 31, 2017 at 20:27

answered Jul 31, 2017 at 20:22

Mike Robinson

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2 Comments

Mike Robinson Over a year ago

(This answer, by the way, has been tested ...)

Anthony Pham Over a year ago

Didn't Gribouillis use regex in his answer though?

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