May 15, 2008 at 1:34PM Quick hacks: Python netstring reader
I’m writing a proof-of-concept server for a less jokey version of the SRGP. The protocol is more or less SRGP, but it doesn’t use MIME-style headers each request is made up of a series of netstrings. The header and body sections are terminated with empty netstrings, i.e., 0:,.
The proof-of-concept server is being written in Python. Here’s the netstring reader generator function, in case it might be of use to somebody else out there:
class MalformedNetstringError:
pass
def netstring_reader(f):
while True:
n = ""
while True:
c = f.read(1)
if c == '':
return
if c == ':':
break
if len(n) > 10:
raise MalformedNetstringError
if c == '0' and n == '':
# We can't allow leading zeros.
if f.read(1) != ':':
raise MalformedNetstringError
n = c
break
n += c
n = int(n, 10)
payload = f.read(n)
if len(payload) < n:
return
if f.read(1) != ',':
raise MalformedNetstringError
yield payload
netstring_reader() takes a file object and returns a generator that returns each netstring extracted from the file object. If the stream is malformed, a MalformedNetstringError is raised.
Currently the diagnostics are far from as good as they ought to be. It should state whether it’s a malformed length or a missing terminating comma, and the offset into the stream where the error occurred. There’s a few places where it can be made more robust, but expect that in later releases.
Here’s an example of it in use.
f = open("sample.dat", "r")
try:
for i in netstring_reader(f):
print i
finally:
f.close()
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