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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def find_invalid_chars(self, text, context_size=20):
 """Find invalid characters in text and store information about the findings. Parameters context_size: int How many characters to return as the context. """

        result = defaultdict(list)
        for idx, char in enumerate(text):
            if char not in self.alphabet:
                start = max(0, idx-context_size)
                end   = min(len(text), idx+context_size)
                result[char].append(text[start:end])
        return result