PyQt5 validator for decimal numbers
I noticed your commenting style and decided it's worthy of it's own answer.

You admit the comma is not always a comma. So don't call it a comma. I went for separator, which is as neutral as it gets. If you think of something better, go for it.

Also, I think your comments would fit better as a docstring than as comments.

class DecimalValidator(QDoubleValidator):
    def __init__(self, *args):
        QDoubleValidator.__init__(self, *args)
        self.setNotation(QDoubleValidator.StandardNotation)
    def validate(self, input, pos):
        '''
        When we're left of the separator, and separator is pressed,
        remove the inserted separator and move right of the old separator.
        When we're right of the separator, and all decimal places are used already,
        go into overwrite mode (by removing the old digit)
        '''
        sep = self.locale().decimalPoint()
        if pos and (input[pos-1]==sep) and (sep in input[pos:]):
            input = input[:pos-1] + input[pos:]
            pos = input.find(sep)+1
        elif sep in input[:pos] and (len(input.split(sep)[1]) &gt; self.decimals()):
            input = input[:pos] + input[pos+1:]
        return QDoubleValidator.validate(self, input, pos)