Add iterator to FlagValue

[polybassa]

This PR adds the iterator functionality to the FlagValue class. This might be useful. At least I want to use it for some OBD related applications.

[codecov-io]

Codecov Report

Merging #1769 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1769      +/-   ##
==========================================
+ Coverage   85.57%   85.58%   +<.01%     
==========================================
  Files         181      181              
  Lines       42187    42200      +13     
==========================================
+ Hits        36103    36117      +14     
+ Misses       6084     6083       -1

Impacted Files	Coverage Δ
scapy/fields.py	91.43% <100%> (+0.08%)	⬆️
scapy/autorun.py	78.44% <0%> (-4.32%)	⬇️
scapy/automaton.py	81.84% <0%> (-0.42%)	⬇️
scapy/layers/tls/record.py	92.1% <0%> (-0.3%)	⬇️
scapy/layers/inet6.py	88.26% <0%> (+0.17%)	⬆️
scapy/asn1/ber.py	82.38% <0%> (+0.28%)	⬆️
scapy/layers/tls/basefields.py	80.53% <0%> (+0.67%)	⬆️
scapy/layers/tls/handshake_sslv2.py	92.77% <0%> (+0.76%)	⬆️
scapy/layers/tls/record_sslv2.py	88.7% <0%> (+1.69%)	⬆️

[p-l-]

Can you explain the purpose of this? Maybe with a concrete example?

[p-l-]

I think __iter__() should return a FlagValueIter object which would have two attributes: the FlagValue instance and a cursor value (which is I think a more meaningful name than iter by the way).

This would make it possible to have several iterators based on the same object with different cursor values at the same time.

[polybassa]

Could you please explain the implementation of the FlagValueIter object more in detail. I would like to implement it to full-fill your remarks. Or can you point me to an example, how this iterator should be implemented.

[p-l-]

Something like that:

class FlagValueIter(object):

    slots = ["value", "cursor"]

    def __init__(self, flagvalue):
        self.value = int(flagvalue)
        self.cursor = 0

    def __iter__(self):
        return self

    def __next__(self):
        x = self.value
        x >>= self.cursor
        while x:
            self.cursor += 1
            if x & 1:
                return self.names[self.cursor - 1]
            x >>= 1
        raise StopIteration

    next = __next__


class FlagValue(object):

    __slots__ = ["value", "names", "multi"]

    [...]

    def __iter__(self):
        return FlagValueIter(self)

Would that be OK for you? It's better to keep the iterator's cursor out of the object, so that you can have multiple simultaneous iterations based on the same object.

[p-l-]

This should be written as: flags = list(a.flags)

[polybassa]

I will change that. Thanks.

[polybassa]

I have such an use case in mind:

resp = sock.sr1(OBD()/OBD_S01(pid=0))
for pid in resp.data_records[0].supportedPIDs:
    req = eval('OBD()/OBD_S01()/OBD_%s()' % pid)
    sock.sr1(req)
    ...

This would allow me to easy generate further request packets based on the FlagsField supportedPIDs.

[polybassa]

@p-l- Is this PR fine, now?

[p-l-]

@polybassa yep! Thank you!