Python核心编程第二版程序14.1改写

#!/usr/bin/env python 
dashes = '\n' + '-' * 50 # dashed line
#dashes用于做格式化输出
exec_dict = { 
'f': """ # for loop 
for %s in %s: 
    print(%s )
""", 
's': """ # sequence while loop 
%s = 0 
%s = %s 
while %s < len(%s): 
    print(%s[%s] ) 
    %s = %s + 1 
""", 
'n': """ # counting while loop 
%s = %d 
while %s < %d: 
    print(%s ) 
    %s = %s + %d 
""" 
} 
def main(): 
    ltype = input('Loop type? (For/While) ').lower() 
    dtype = input('Data type? (Number/Seq) ').lower() 
    #输入f或者w
    if dtype == 'n': 
        start = int(input('Starting value? ') ) 
        stop = int(input('Ending value (non-inclusive)? ') )
        step = int(input('Stepping value? ') )
        seq = str(range(start, stop, step)) 
    else: 
        seq = input('Enter sequence: ') 
    var = input('Iterative variable name? ') 
    if ltype == 'f': 
        exec_str = exec_dict['f'] % (var, seq, var) 
    elif ltype == 'w': 
        if dtype == 's': 
            svar = input('Enter sequence name? ') 
            exec_str = exec_dict['s'] % \
            (var, svar, seq, var, svar, svar, var, var, var) 
        elif dtype == 'n': 
            exec_str = exec_dict['n'] % \
            (var, start, var, stop, var, var, var, step) 
    print(dashes) 
    #读取之后开始拼接字符串输出
    print('Your custom-generated code:' + dashes )
    print(exec_str + dashes )
    print('Test execution of the code:' + dashes )
    exec(exec_str )
    print(dashes )
if __name__ == '__main__': 
    main()