How to read multiple files with error checking?

Question

I have a function that reads multiple files. Like this:

try:
    a = open("1.txt", "r")
    b = open("2.txt", "r")
    c = open("3.txt", "r")
except IOError:
    print("File {:s} failed".format(a, b or c))

I want that I can see which file failed during reading it. Can I somehow specify the IOError for specified file? I mean that if IOError appears in file a, do "command1", if in file b, do "command2" and so on?

Answer 1

The IOError exception is an alias of the OSError exception, which has a filename attribute. You can use that to switch behaviour based on the file that failed:

try:
    a = open("1.txt", "r")
    b = open("2.txt", "r")
    c = open("3.txt", "r")
except OSError as error:
    print("File {:s} failed".format(error.filename))

I used the name OSError; IOError is deprecated and only kept around for backwards compatibility, see PEP 3151.

Demo:

>>> try:
...     open('Nonesuch.txt')
... except OSError as error:
...     print('File {:s} failed'.format(error.filename))
... 
File Nonesuch.txt failed

Note that it is the open() call that throws the exception, so no assignment has taken place. And because a file object can be referenced from multiple places, including from a list, there is no way to map a file object or filename back to the name you were going to assign it to. If you wanted to know which of a, b or c the file object would have been bound to, you'd have to create your own mapping:

filenames = {'1.txt': 'a', '2.txt': 'b', '3.txt': 'c'}
print("File {} failed".format(filenames[error.filename]))