Decode the Message LeetCode solution in Python

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class Solution:
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    def decodeMessage(self, key: str, message: str) -> str:
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        mapping = {" ": " "}
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        i = 0
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        res = ""
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        letters = "abcdefghijklmnopqrstuvwxyz"
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        for char in key:
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            if char not in mapping:
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                mapping[char] = letters[i]
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                i += 1
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        for char in message:
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            res += mapping[char]
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        return res