Use os.access(...) to check if the user is allowed to read/write that file:

if os.access(os.R_OK) ... # read ok

if os.access(os.W_OK) ... # write ok

if os.access(os.R_OK | os.W_OK) ... # read and write ok

https://docs.python.org/2.7/library/os.html#os.access

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