python变量不能以数字打头

在编写python函数时，无意中发现一个问题：python中的变量不能以数字打头，以下函数中定义了一个变量3_num_varchar，执行时报错。

函数如下：

def database_feild_varchar_trans(in_feild):
    '''
    transfer the feild if varchar then 3times lang else no transfer
    '''
    feild_split = in_feild.split(' ')
    is_varchar = feild_split[1].find('VARCHAR')
    if is_varchar >= 0 :
       num_varchar = feild_split[1].replace('VARCHAR','').replace('(','').replace(')','')
       print (num_varchar)
       3_num_varchar = num_varchar*3
       feild_split[1] =  feild_split[1].replace(str(num_varchar),str(3_num_varchar))
       return feild_split
    else:
       print ('The feild type is not varchar')
       return feild_split
报错信息如下：

>>> runfile('E:/procedure/python/projects/others/table_test.py', wdir='E:/procedure/python/projects/others')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "D:\Python33\lib\site-packages\spyderlib\widgets\externalshell\sitecustomize.py", line 699, in runfile
    execfile(filename, namespace)
  File "D:\Python33\lib\site-packages\spyderlib\widgets\externalshell\sitecustomize.py", line 88, in execfile
    exec(compile(open(filename, 'rb').read(), filename, 'exec'), namespace)
  File "E:/procedure/python/projects/others/table_test.py", line 20
    3_num_varchar = int(num_varchar)*3
                ^
SyntaxError: invalid syntax

将变量3_num_varchar改为num_varchar_3，运行成功，程序改为如下：

import os
import sys
str1='aaa varchar(10)'

def database_feild_varchar_trans(in_feild):
    '''
    transfer the feild if varchar then 3times lang else no transfer
    '''
    feild_split = in_feild.split(' ')
    is_varchar = feild_split[1].find('VARCHAR')
    if is_varchar >= 0 :
       num_varchar = feild_split[1].replace('VARCHAR','').replace('(','').replace(')','')
       print (num_varchar)
       num_varchar_3 = num_varchar*3
       feild_split[1] =  feild_split[1].replace(str(num_varchar),str(num_varchar_3))
       return feild_split
    else:
       print ('The feild type is not varchar')
       return feild_split

print (database_feild_varchar_trans(str1))

运行结果：

>>> runfile('E:/procedure/python/projects/others/table_test.py', wdir='E:/procedure/python/projects/others')
The feild type is not varchar
['aaa', 'varchar(10)']