python怎么把字符串转化成数字

>>> s1='123'

>>> s2='1.23'

>>> s3=''

>>> s4=None

>>> int(s1)123

>>> int(s2)ValueError

>>> int(s3)ValueError

>>> int(s4)TypeError

>>> float(s1)123.0

>>> float(s2)1.23

>>> float(s3)ValueError

>>> float(s4)TypeError

>>> float('1e3')1000.0

def str_to_float( s):

  """字符串转换为float"""

  if s is None:

    return 0.0

    try:

      return float(s)

    except Exception:

      return 0.0

>>> s='12%'

>>> float(s.rstrip('%'))/1000.12

>>> import unicodedata

>>> unicodedata.numeric('三')3.0

>>> unicodedata.numeric('二十一')TypeError: numeric() argument 1 must be a unicode character, not str