def convert_to_float(frac_str):
    try:
        return float(frac_str)
    except ValueError:
        num, denom = frac_str.split('/')
        try:
            leading, num = num.split(' ')
            whole = float(leading)
        except ValueError:
        	whole = 0
        frac = float(num) / float(denom)
        return whole - frac if whole < 0 else whole + frac
 
 
print convert_to_float('3')
print convert_to_float('3/2')
print convert_to_float('1 1/2')
print convert_to_float('-1 1/2')

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