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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 fast_float( x, key=lambda x: x, nan=None, _uni=unicodedata.numeric, _nan_inf=NAN_INF, _first_char=POTENTIAL_FIRST_CHAR, ):
 """ Convert a string to a float quickly, return input as-is if not possible. We don't need to accept all input that the real fast_int accepts because natsort is controlling what is passed to this function. Parameters x : str String to attempt to convert to a float. key : callable Single-argument function to apply to *x* if conversion fails. nan : object Value to return instead of NaN if NaN would be returned. Returns ------- *str* or *float* """

    if x[0] in _first_char or x.lstrip()[:3] in _nan_inf:
        try:
            x = float(x)
            return nan if nan is not None and x != x else x
        except ValueError:
            try:
                return _uni(x, key(x)) if len(x) == 1 else key(x)
            except TypeError:  # pragma: no cover
                return key(x)
    else:
        try:
            return _uni(x, key(x)) if len(x) == 1 else key(x)
        except TypeError:  # pragma: no cover
            return key(x)