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Python

Posted by Kyle Calica
Kyle Calica
5,955 Points

from_string: A weird issue where b is 'dot' won't run conditional but b == 'dot' will?

I have this weird issue where my code works with == in the if statements but not with is but I've used this is before for check '.' and '_' in an earlier answer:

"-".join(['dot' if p is '.' else 'dash' if p is '-' else '' for p in pattern].strip(''))

Even prints out both are true. Do I have some error in my format? I want to get this to another one-liner using is as it's more pythonic that way.

morse.py 
class Letter:
    def __init__(self, pattern=None):
        self.pattern = pattern

    def __iter__(self):
        yield from self.pattern

    def __str__(self):
        output = []
        for blip in self:
            if blip == '.':
                output.append('dot')
            else:
                output.append('dash')
        return '-'.join(output)

    @classmethod
    def from_string(cls, string):
        #return cls( [ '.' if b is 'dot' else '_' if b is 'dash' else '' for b in string.lower().split('-') ]  ) 
        blips = string.lower().split('-')
        marks = []
        for b in blips:
            if b is 'dot':
                marks.append('.')
            if b is 'dash':
                marks.append('_')
        return cls(marks)



class S(Letter):
    def __init__(self):
        pattern = ['.', '.', '.']
        super().__init__(pattern)

1 Answer

Steven Parker
Steven Parker
231,248 Points
Steven Parker
Steven Parker
231,248 Points

Actually, that's not a proper use of "is". The "==" operator is a test for value equality, which is the correct one for this situation. The "is" operator is a test for identity.

The Identity test only worked because of a technicality in the way Python stores string literals for efficiency, definitely not something that the code should rely on!