Here is my code (py3.1):
def get_string(self,chars):
return struct.unpack("{}s".format(chars),self.get_bin(chars))
b'MESH' is going in, (b'MESH',) is coming out. Why am I not getting a string?
3 Answers
It helps when asking questions if you make sure that the code is actually what you did run. What you show would have given an error. Also ensure that the code snippet includes all information so that people don't need to guess.
>>> chars = "MESH" # guess
>>> binchars = b"MESH" # guess
>>> struct_fmt = "{}s".format(chars) # what you showed
>>> struct_fmt
'MESHs' ############### won't work
>>> import struct
>>> struct.unpack(struct_fmt, binchars)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
struct.error: bad char in struct format ############ didn't work
>>> struct_fmt2 = "{}s".format(len(chars)) # what you probably did use
>>> struct_fmt2
'4s' #### looks much better
>>> struct.unpack(struct_fmt2, binchars)
(b'MESH',) ### did work
>>>
As the manual says, "For unpacking, the resulting bytes object ..."
2 Comments
Artfunkel
I did post my code and it does work, thank you.
chars is the number of chars in the desired string, not the string itself like you "guessed". (It does look like you answered the question though. So I forgive you.)
John Machin
chars is not an appropriate name for an integer, the format (
{}) gave no clue at all. You didn't show the WHOLE code (self.get_bin(chars) ???) so that somebody could reproduce what you were doing. In fact, all you needed to show to demonstrate what you meant was struct.unpack("4s", b"MESH")This works:
return struct.unpack("{}s".format(chars),self.get_bin(chars))[0].decode('ASCII')
But does not explain why unpack() neglects to do this itself. I'll leave the question open until that's answered.
Edit: Jason's improvement:
return self.get_bin(chars).decode('ASCII')
Makes a mockery of struct really...
3 Comments
Joe Kington
I'm confused... Why would you expect
struct to decode a string?? And even if struct would be expected to do this, how would struct.unpack be able to automatically infer whether the string is ASCII or UTF8 or some other type of unicode?Artfunkel
10s > char[10] and 10w > wchar_t[10]? I'm assuming there is actually a way, as it does claim to do string.John Machin
"makes a mockery": well of course if that's all you have, you don't don't need
struct ... struct is for structures that might require a format like "<4s2H2Bd"unpack returns a tuple, a tuple of length 1 in this case.
Add
[0]
to the end of what you have and it should give what you expect.
bytesinto into astr, you are doing it wrong(tm).structis for dealing with tightly-laid-out binary(!) data.