How note a wrong password with Python's UnRAR library?

Question

The following code (which tries to “open” an encrypted RAR-file supplying a wrong password):

from unrar import rarfile
import unrar 

try:
    rarfile.RarFile("encrypted.rar", pwd="wrong_password")
except Exception as e:
    print(type(e))
    print(e)

mostly, though nothing else is wrong with the RAR-file (can be decrypted without errors by using the correct password), outputs:

<class 'unrar.rarfile.BadRarFile'>
Invalid RAR file.

but sometimes it outputs:

<class 'RuntimeError'>
Bad password for Archive

How do I check if a password for a RAR-file is correct with Python's UnRAR library without chaining the exceptions?

In short: UnRAR library raises (randomly?) different exception for the same type of error (namely, wrong password supplied). In most of the cases it raises BadRarFile but sometimes it raises RuntimeError . Catching RuntimeError is bad enough (yet here we can at least check the args ), but if one also catches except unrar.rarfile.BadRarFile , one cannot even differentiate between the error that (a) the password is wrong or (b) that the RAR-file is bad.

Answer 1

You could chain multiple except to narrow down the error. Unfortunately, your unrar library seems to raise the unspecific exception RuntimeError in case a bad password is provided. So you cannot be 100% sure if a bad password is the reason for the error.

try:
    unrar.rarfile.RarFile("encrypted.rar", pwd="wrong_password")
except unrar.rarfile.BadRarFile:
    print("Specified file doesn't seem to be a proper RAR archive")
except RuntimeError:
    print("RuntimeError, possibly a wrong password")
except:
    print("Something else happened")

Other than using different error messages "Wrong password or defective file" and "Wrong password or something else", unfortunately, I don't see any possibility for improvement.

In short: UnRAR library raises (randomly?) different exception for the same type of error (namely, wrong password supplied). In most of the cases it raises BadRarFile but sometimes it raises RuntimeError .

It's possible that depending on the version of the RAR file specs, there have been changes how a bad password is being handled. Maybe it isn't possible to differentiate between corrupt files and a wrong password with RAR files of a newer version while this was possible for older files. (Or the other way around.)

If the "original" unrar command doesn't have this issue, it's possibly a bug upstream in your Python wrapper library.