How to make a fixed-size byte variable in Python
Solution 1
For assembling packets to go over the network, or for assembling byte-perfect binary files, I suggest using the struct
module.
Just for the string, you might not need struct
, but as soon as you start also packing binary values, struct
will make your life much easier.
Depending on your needs, you might be better off with an off-the-shelf network serialization library, such as Protocol Buffers; or you might even just use JSON for the wire format.
Solution 2
Something like this should work:
st = "具有"
by = bytes(st, "utf-8")
by += b"0" * (100 - len(by))
print(by)
# b'\xe5\x85\xb7\xe6\x9c\x890000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000'
Obligatory addendum since your original post seems to conflate strings with the length of their encoded byte representation: Python unicode explanation
Solution 3
To pad with null bytes you can do it the way they do it in the stdlib base64 module.
some_data = b'foosdsfkl\x05'
null_padded = some_data + bytes(100 - len(some_data))
Solution 4
Here's a roundabout way of doing it:
>>> import sys
>>> a = "a"
>>> sys.getsizeof(a)
22
>>> a = "aa"
>>> sys.getsizeof(a)
23
>>> a = "aaa"
>>> sys.getsizeof(a)
24
So following this, an ASCII string of 100 bytes will need to be 79 characters long
>>> a = "".join(["a" for i in range(79)])
>>> len(a)
79
>>> sys.getsizeof(a)
100
This approach above is a fairly simple way of "calibrating" strings to figure out their lengths. You could automate a script to pad a string out to the appropriate memory size to account for other encodings.
def padder(strng):
TARGETSIZE = 100
padChar = "0"
curSize = sys.getsizeof(strng)
if curSize <= TARGETSIZE:
for i in range(TARGETSIZE - curSize):
strng = padChar + strng
return strng
else:
return strng # Not sure if you need to handle strings that start longer than your target, but you can do that here
Mikael S.
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Updated on June 27, 2022Comments
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Mikael S. almost 2 years
Let's say, I have a string (Unicode if it matters) variable which is less than 100 bytes. I want to create another variable with exactly 100 byte in size which includes this string and is padded with zero or whatever. How would I do it in Python 3?