How to check if X-quacks are like list / tuple
How do we check if X-quacks are like a list / tuple?
By that, I mean we can multiply it by 0 and 1 (etc.), although it cannot be a string (which can also be a subset).
I thought to test hasattr(X, '__iter__')
and not isinstance(X, str)
, but that would mean that the dictionary will still pass, which I don't want to. Then you can also check that it is not a dictionary, but I would not be so sure about the meanness of dicts etc.
Is there a more official way to check that something does the same as a list or tuple according to this simple specification?
eg. a valid input must be:
'emailaddr' --> ('emailaddr', 'emailaddr')
('emailaddr', 'alias')
['emailaddr', 'alias']
SequenceThing('emailaddr', 'alias')
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Do you want to check if the value is sequence ?
An iterable that supports efficient access to an element using integer indices using the special getitem () method and defines a len () method that returns the length of the sequence. Some built-in sequence types are list, str, tuple, and bytes. Note that dict also supports getitem () and len (), but is considered a match rather than a sequence because the searches use arbitrary immutable keys rather than integers.
Old way (deprecated in 2.7):
import operator
print operator.isSequenceType(x)
New way:
import collections
print isinstance(x, collections.Sequence)
They are not equivalent - the latter are more precise, but user-defined types must be registered with it.
In addition, strings are sequences according to both definitions. You can use basestring
specifically.
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Don't allow a bare line as an argument; this is a special case waiting to bite you. Instead, document that the input must be a tuple / list / whatever, but the second element is optional. I.e
['emailaddr', ]
('emailaddr', )
SequenceThing('emailaddr')
are legal but not
'emailaddr'
So your function might assume it X[0]
would be an email address, or it X[1]
might exist and be an alias, but that's a user's problem if they supply anything else.
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Solution based on Kevin's helpful comment:
>>> import collections
>>> isinstance([], collections.Sequence) and not isinstance([], str)
True
>>> isinstance((), collections.Sequence) and not isinstance((), str)
True
>>> isinstance('', collections.Sequence) and not isinstance('', str)
False
>>> isinstance({}, collections.Sequence) and not isinstance({}, str)
False
>>> isinstance(set(), collections.Sequence) and not isinstance(set(), str)
False
Mappings such as dicts and sets will fail the first test. There will be lines, but they will obviously fail in the second test.
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