Crypto.py Function check_and_join

[aditya.ka...@itbhu.ac.in]

Hi All,
I was reading sympy crypto library where I am unable to understand something in 'check_and_join' function. 

def check_and_join(phrase, symbols=None, filter=None):

    """

    Joins characters of `phrase` and if ``symbols`` is given, raises

    an error if any character in ``phrase`` is not in ``symbols``.

    Parameters

    ==========

    phrase:     string or list of strings to be returned as a string

    symbols:    iterable of characters allowed in ``phrase``;

                if ``symbols`` is None, no checking is performed

    Examples

    ========

    >>> from sympy.crypto.crypto import check_and_join

    >>> check_and_join('a phrase')

    'a phrase'

    >>> check_and_join('a phrase'.upper().split())

    'APHRASE'

    >>> check_and_join('a phrase!'.upper().split(), 'ARE', filter=True)

    'ARAE'

    >>> check_and_join('a phrase!'.upper().split(), 'ARE')

    Traceback (most recent call last):

    ...

    ValueError: characters in phrase but not symbols: "!HPS"

    """

    rv = ''.join(''.join(phrase))

    if symbols is not None:

        symbols = check_and_join(symbols)

        missing = ''.join(list(sorted(set(rv) - set(symbols))))

        if missing:

            if not filter:

                raise ValueError(

                    'characters in phrase but not symbols: "%s"' % missing)

            rv = translate(rv, None, missing)

    return rv

This maybe very silly but use of join function twice is not clear to me. Please if anyone could help. :) 
Thanks in advance.
Aditya

[Aaron Meurer]

Seems useless to me. ''.join(s) should return s if s is a string.

Aaron Meurer

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