Dabeaz

When I first saw the new Python 3 bytearray object (also back-ported to Python 2.6), I wasn't exactly sure what to make of it. On the surface, it seemed like a kind of mutable 8-bit string (a feature sometimes requested by users of Python 2). For example:

>>> s = bytearray(b"Hello World")
>>> s[:5] = b"Cruel"
>>> s
bytearray(b'Cruel World')
>>>

On the other hand, there are aspects of bytearray objects that are completely unlike a string. For example, if you iterate over a bytearray, you get integer byte values:

>>> s = bytearray(b"Hello World")
>>> for c in s: print(c)
...
72
101
108
108
111
32
87
111
114
108
100
>>> 

Similarly, indexing operations are tied to integers:

>>> s[1]
101
>>> s[1] = 97
>>> s[1] = b'a'
Traceback (most recent call last):
  File "", line 1, in 
TypeError: an integer is required
>>> 

Finally, there's the fact bytearray instances have most of the methods associated with strings as well as methods associated with lists. For example:

>>> s.split()
[bytearray(b'Hello'), bytearray(b'World')]
>>> s.append(33)
>>> s
bytearray(b'Hello World!')
>>>

Although documentation on bytearrays describes these features, it is a little light on meaningful use cases. Needless to say, if you have too much spare time (sic) on your hands, this is the kind of thing that you start to think about. So, I thought I'd share three practical uses of bytearrays.

Example 1: Assembling a message from fragments

Suppose you're writing some network code that is receiving a large message on a socket connection. If you know about sockets, you know that the recv() operation doesn't wait for all of the data to arrive. Instead, it merely returns what's currently available in the system buffers. Therefore, to get all of the data, you might write code that looks like this:

# remaining = number of bytes being received (determined already)
msg = b""
while remaining > 0:
    chunk = s.recv(remaining)    # Get available data
    msg += chunk                 # Add it to the message
    remaining -= len(chunk)  

The only problem with this code is that concatenation (+=) has horrible performance. Therefore, a common performance optimization in Python 2 is to collect all of the chunks in a list and perform a join when you're done. Like this:

# remaining = number of bytes being received (determined already)
msgparts = []
while remaining > 0:
    chunk = s.recv(remaining)    # Get available data
    msgparts.append(chunk)       # Add it to list of chunks
    remaining -= len(chunk)  
msg = b"".join(msgparts)          # Make the final message

Now, here's a third solution using a bytearray:

# remaining = number of bytes being received (determined already)
msg = bytearray()
while remaining > 0:
    chunk = s.recv(remaining)    # Get available data
    msg.extend(chunk)            # Add to message
    remaining -= len(chunk)  

Notice how the bytearray version is really clean. You don't collect parts in a list and you don't perform that cryptic join at the end. Nice.

Of course, the big question is whether or not it performs. To test this out, I first made a list of small byte fragments like this:

chunks = [b"x"*16]*512

I then used the timeit module to compare the following two code fragments:

# Version 1
msgparts = []
for chunk in chunks:
    msgparts.append(chunk)
msg = b"".join(msgparts)

# Version 2
msg = bytearray()
for chunk in chunks:
    msg.extend(chunk)

When tested, version 1 of the code ran in 99.8s whereas version 2 ran in 116.6s (a version using += concatenation takes 230.3s by comparison). So while performing a join operation is still faster, it's only faster by about 16%. Personally, I think the cleaner programming of the bytearray version might make up for it.

Example 2: Binary record packing

This example is an slight twist on the last example. Support you had a large Python list of integer (x,y) coordinates. Something like this:

points = [(1,2),(3,4),(9,10),(23,14),(50,90),...]

Now, suppose you need to write that data out as a binary encoded file consisting of a 32-bit integer length followed by each point packed into a pair of 32-bit integers. One way to do it would be to use the struct module like this:

import struct
f = open("points.bin","wb")
f.write(struct.pack("I",len(points)))
for x,y in points:
    f.write(struct.pack("II",x,y))
f.close()

The only problem with this code is that it performs a large number of small write() operations. An alternative approach is to pack everything into a bytearray and only perform one write at the end. For example:

import struct
f = open("points.bin","wb")
msg = bytearray()
msg.extend(struct.pack("I",len(points))
for x,y in points:
    msg.extend(struct.pack("II",x,y))
f.write(msg)
f.close()

Sure enough, the version that uses bytearray runs much faster. In a simple timing test involving a list of 100000 points, it runs in about half the time as the version that makes a lot of small writes.

Example 3: Mathematical processing of byte values

The fact that bytearrays present themselves as arrays of integers makes it easier to perform certain kinds of calculations. In a recent embedded systems project, I was using Python to communicate with a device over a serial port. As part of the communications protocol, all messages had to be signed with a Longitudinal Redundancy Check (LRC) byte. An LRC is computed by taking an XOR across all of the byte values.

Bytearrays make such calculations easy. Here's one version:

message = bytearray(...)     # Message already created
lrc = 0
for b in message:
    lrc ^= b
message.append(lrc)          # Add to the end of the message

Here's a version that increases your job security:

message.append(functools.reduce(lambda x,y:x^y,message))

And here's the same calculation in Python 2 without bytearrays:

message = "..."       # Message already created
lrc = 0
for b in message:
    lrc ^= ord(b)
message += chr(lrc)        # Add the LRC byte

Personally, I like the bytearray version. There's no need to use ord() and you can just append the result at the end of the message instead of using concatenation.

Here's another cute example. Suppose you wanted to run a bytearray through a simple XOR-cipher. Here's a one-liner to do it:

>>> key = 37
>>> message = bytearray(b"Hello World")
>>> s = bytearray(x ^ key for x in message)
>>> s
bytearray(b'm@IIJ\x05rJWIA')
>>> bytearray(x ^ key for x in s)
bytearray(b"Hello World")
>>> 

Final Comments

Although some programmers might focus on bytearrays as a kind of mutable string, I find their use as an efficient means for assembling messages from fragments to be much more interesting. That's because this kind of problem comes up a lot in the context of interprocess communication, networking, distributed computing, and other related areas. Thus, if you know about bytearrays, it might lead to code that has good performance and is easy to understand.

That's it for this installment. In case you're wondering, this topic is also related to my upcoming PyCON'2010 tutorial "Mastering Python 3 I/O."

posted by Dave  # 7:02 PM