Why the following two decoding methods return different results?

>>> import codecs
>>>
>>> data = ['', '', 'a', '']
>>> list(codecs.iterdecode(data, 'utf-8'))
[u'a']
>>> [codecs.decode(i, 'utf-8') for i in data]
[u'', u'', u'a', u'']

Is this a bug or expected behavior? My Python version 2.7.13.

This is normal. iterdecode takes an iterator over encoded chunks and returns an iterator over decoded chunks, but it doesn't promise a one-to-one correspondence. All it guarantees is that the concatenation of all output chunks is a valid decoding of the concatenation of all input chunks.

If you look at the source code, you'll see it's explicitly discarding empty output chunks:

def iterdecode(iterator, encoding, errors='strict', **kwargs):"""Decoding iterator.Decodes the input strings from the iterator using an IncrementalDecoder.errors and kwargs are passed through to the IncrementalDecoderconstructor."""decoder = getincrementaldecoder(encoding)(errors, **kwargs)for input in iterator:output = decoder.decode(input)if output:yield outputoutput = decoder.decode("", True)if output:yield output

Be aware that the reason iterdecode exists, and the reason you wouldn't just call decode on all the chunks yourself, is that the decoding process is stateful. The UTF-8 encoded form of one character might be split over multiple chunks. Other codecs might have really weird stateful behavior, like maybe a byte sequence that inverts the case of all characters until you see that byte sequence again.