How to use 'balanced parentheses in python' in Python

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def close_parens_needed(expr):
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    """Return the number of left-parentheses needed to make 'expr'
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       balanced.
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    >>> close_parens_needed("1+2")
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    0
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    >>> close_parens_needed("(1 + 2)")
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    0
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    >>> close_parens_needed("(1 + 2")
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    1
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    >>> close_parens_needed("(1 + (2 *")
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    2
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    >>> close_parens_needed("(1 + (2 * 3) + (4")
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    2
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    """
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    return expr.count("(") - expr.count(")")