help with explaining how to split a list of tuples into parts

peter at ifoley.id.au peter at ifoley.id.au
Sat Jul 13 08:43:55 CEST 2013


Hi List,

I am new to Python and wondering if there is a better python way to do something.  As a learning exercise I decided to create a python bash script to wrap around the Python Crypt library (Version 2.7).

My attempt is located here - https://gist.github.com/pjfoley/5989653

I am trying to wrap my head around list comprehensions, I have read the docs at http://docs.python.org/2/tutorial/datastructures.html#list-comprehensions and read various google results.  I think my lack of knowledge is making it difficult to know what key word to search on.

Essentially I have this list of tuples

# Tuple == (Hash Method, Salt Length, Magic String, Hashed Password Length)
supported_hashes=[('crypt',2,'',13), ('md5',8,'$1$',22), ('sha256',16,'$5$',43), ('sha512',16,'$6$',86)]

This list contains the valid hash methods that the Crypt Library supports plus some lookup values I want to use in the code.

I have managed to work out how to extract a list of just the first value of each tuple (line 16) which I use as part of the validation against the --hash argparse option.

My Question.

Looking at line 27, This line returns the tuple that mataches the hash type the user selects from the command line.  Which I then split the seperate parts over lines 29 to 31.

I am wondering if there is a more efficient way to do this such that I could do:

salt_length, hash_type, expected_password_length = [x for x in supported_hashes if x[0] == args.hash]

>From my limited understanding the first x is the return value from the function which meets the criteria.  So could I do something like:

... = [(x[0][1], x[0][2], x[0][3]) for x in supported_hashes if x[0] == args.hash]

I am happy to be pointed to some documentation which might help clarify what I need to do.  

Also if there is anything else that could be improved on with the code happy to be contacted off list.

Thanks,

Peter.



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