Identify the number of elements in a python struct/pack fmt string?

Question

When using the python struct module on can specify a format string that declares how binary data should be interpreted:

>>> from struct import *
>>> fmt = 'hhl'
>>> values = [1,2,3]
>>> blob = pack(fmt, values)

It is easily possible to calculate the amount of bytes needed to store an instance of that format:

>>> calcsize(fmt)

What would be the best way to retrieve the number of variables need to 'fill' a format? Basically this would tell in advance how big the 'values' array should be to perform the pack() in the above example.

>>> calcentries(fmt)
3

Is there such a thing?

score 5 · Accepted Answer · answered Mar 13 '11 at 11:44

5

I'm afraid there's no such function in the struct API, but you can define it yourself without parsing the format string:

def calcentries(fmt):
    return len(struct.unpack(fmt, '\0' * struct.calcsize(fmt)))

answered Mar 13 '11 at 11:44

Fred Foo

355,277
75
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1

Nice, thanks. Actually, this probably does parse the format string twice. In the actual code I'm going to cache the result. – dantje Mar 13 '11 at 11:53
@dantje: I meant without implementing the parsing yourself. – Fred Foo Mar 13 '11 at 11:54
1

Yeah, understood :-) I continue to be amazed how nice things can be expressed in python. – dantje Mar 13 '11 at 12:04

Identify the number of elements in a python struct/pack fmt string?

1 Answers1