python - 如何在提供有效文件名后退出 for 循环并给出 3 次正确输入的机会

Question

我正在尝试编写一个 for 循环，让用户 3 次尝试提供正确的文件名。执行此操作的最佳方法或最佳实践是什么？

for n in [1,2,3]:
    try:
        user_inp = input('Please provide the filename: ')  # Prompt user for file name
    # Need help here..
    # If the filename isn't found, python will give the following error: FileNotFoundError: [Errno 2] No such file or directory: 
    # If that happens, I need it to print the except statement and try again 
    # However, if the filename is correct or "FileIsFound" I need it to exit the loop--using ' break ' ?         
    except:
          print('Invalid file name, try again please')
    # Would be great if we could add a counter here like: print('You have', n, 'tries remaining') 
    continue

file_open = open(user_inp) # open file --load in memory 

for line in file_open:  # remove /n at the end of each line
line = line.rstrip()  # print statements add /n = newlines and text docs usually have /n in them
# this for loop will remove/strip the /n at the end of each line

read_file = file_open.read()  # reads entire file 
                              # NOTE: if you are dealing with large files, you may want to consider using a conditional statement to find what you are searching, extracting, etc instead of reading the entire file


print(read_file.upper())  # Prints file contents in upper case 

我想知道如何使用 try 和 except 语句来执行此操作。谢谢!

干杯。

Answer 1

操作系统模块

使用 os 模块中的 isfile() 方法检查文件是否存在。

演示:

>>> import os
>>> os.path.isfile("/1.txt")
False
>>> os.path.isfile("/home/vivek/Desktop/file3.csv")
True
>>> 

中断关键字

使用 break 关键字从任何迭代器循环中退出，即 for、while

演示

>>> c = 1
>>> while True:
...     if c==3:
...         print "Now break because value of c (%d) is equal to 3"%(c)
...         break
...     print "In loop c=%d"%c
...     c += 1
... 
In loop c=1
In loop c=2
Now break because value of c (3) is equal to 3
>>> 

<小时/>

从用户获取文件名并验证文件名的代码。

import os

file_path = ""
for n in [1,2,3]:
    user_inp = raw_input('Please provide the filename: ')
    if os.path.isfile(user_inp):
        file_path = user_inp
        break 
    print('Invalid file name, try again please')


if file_path:
    # Write ypur code.
    print "Processing file Name:", file_path
else:
    print "User not enters correct file name."

输出:

$ python test1.py
Please provide the filename: /home/Desktop/1.txt
Invalid file name, try again please
Please provide the filename: /home/Desktop/2.txt                
Invalid file name, try again please
Please provide the filename: /home/Desktop/3.txt            
Invalid file name, try again please
User not enters correct file name.

$ python test1.py
Please provide the filename: /home/Desktop/2.txt
Invalid file name, try again please
Please provide the filename: /home/vivek/Desktop/file3.csv
Processing file Name: /home/vivek/Desktop/file3.csv

<小时/>

注意:

对 Python 2.x 使用 raw_input()

对 Python 3.x 使用 input()