Check if file is readable with Python: try or if/else?

Question

I have the following code:

import glob, os
for file in glob.glob("\\*.txt"):
    if os.access(file, os.R_OK):
        # Do something
    else:
        if not os.access(file, os.R_OK):
            print(file, "is not readable")
        else:
            print("Something went wrong with file/dir", file)
        break

But I'm not entirely sure if this the right way to do it. Is it better to use try and catch the error? If so, how do I try for readability? Note the break in my else statement. As soon as a file can't be read I want to abort the loop.

Answer 1

A more explicit way to check if file is actually a file and not directory for example, and it is readable:

from os import access, R_OK from os.path import isfile  file = "/some/path/to/file"  assert isfile(file) and access(file, R_OK), \        f"File {file} doesn't exist or isn't readable" 

Answer 2

For me, using a try-except at the same scope as one would use an if-else gains no readability. The value of exceptions is that they can be caught at a higher level in the call tree.

Moving out just one level, we avoid the break statement:

import glob, os
try:
    for file in glob.glob("\\*.txt"):
        with open(file) as fp:
            # do something with file
except IOError:
    print("could not read", file)

But the true genius of exceptions is when the code simply disappears:

# Operate on several files
# SUCCESS: Returns None
# FAIL: Raises exception
def do_some_files():
    for file in glob.glob("\\*.txt"):
        with open(file) as fp:
            # do something with file

Now it is the calling program's responsibility to display a useful error message on failure. We have removed responsibility for dealing with failure completely out of this code and into a whole other realm.

In fact, one can move the responsibility completely out of our program and into the interpreter. In that case, the interpreter will print some useful error message and terminate our program. If Python's default message is good enough for your users, I recommend not checking for errors at all. Thus, your original script becomes:

import glob, os
for file in glob.glob("\\*.txt"):
    # Do something