On 20 апр, 04:10, George Sakkis <[EMAIL PROTECTED]> wrote: > On Apr 18, 9:36 pm, Ross Ridge <[EMAIL PROTECTED]> > wrote: > > > > > Ross Ridge <[EMAIL PROTECTED]> said: > > > > If you have Python 2.5, here's a faster version: > > > > from struct import * > > > unpack_i32be = Struct(">l").unpack > > > > def from3Bytes_ross2(s): > > > return unpack_i32be(s + "\0")[0] >> 8 > > > Bob Greschke <[EMAIL PROTECTED]> wrote: > > > > That's not even intelligible. I wanna go back to COBOL. :) > > > It's the same as the previous version except that it "precompiles" > > the struct.unpack() format string. It works similar to the way Python > > handles regular expressions. > > I didn't know about the Struct class; pretty neat. It's amazing that > this version without Psyco is as fast Bob's version with Psyco! Adding > Psyco to it though makes it *slower*, not faster. So here's how I'd > write it (if I wanted or had to stay in pure Python): > > try: import psyco > except ImportError: > from struct import Struct > unpack_i32be = Struct(">l").unpack > def from3Bytes(s): > return unpack_i32be(s + "\0")[0] >> 8 > else: > def from3Bytes(s): > Value = (ord(s[0])<<16) + (ord(s[1])<<8) + ord(s[2]) > if Value >= 0x800000: > Value -= 0x1000000 > return Value > psyco.bind(from3Bytes) > > HTH, > George
I was able to get even faster pure-python version using array module: a = array.array('l', ''.join(('\0' + s[i:i+3] for i in xrange(0, len(s), 3)))) if sys.byteorder == 'little': a.byteswap() It actually moves bytes around on C level. test code: import struct import array import sys unpack_i32be = struct.Struct(">l").unpack s = ''.join(struct.pack('>i', 1234567)[1:]*1000) def from3bytes_ord(s): values = [] for i in xrange(0, len(s), 3): Value = (ord(s[i])<<16) | (ord(s[i+1])<<8) | ord(s[i+2]) if Value >= 0x800000: Value -= 0x1000000 values.append(Value) return values def from3bytes_struct(s): return [unpack_i32be(s[i:i+3] + "\0")[0] >> 8 for i in xrange(0, len(s), 3)] def from3bytes_array(s): a = array.array('l', ''.join(('\0' + s[i:i+3] for i in xrange(0, len(s), 3)))) if sys.byteorder == 'little': a.byteswap() return a.tolist() from timeit import Timer t1 = Timer("from3bytes_ord(s)", "from __main__ import s, from3bytes_ord") t2 = Timer("from3bytes_struct(s)", "from __main__ import s, from3bytes_struct") t3 = Timer("from3bytes_array(s)", "from __main__ import s, from3bytes_array") print 'ord:\t', t1.timeit(1000) print 'struct:\t', t2.timeit(1000) print 'array:\t', t3.timeit(1000) Output: ord: 7.08213110884 struct: 3.7689164405 array: 2.62995268952 Inspired by Guido's essay http://www.python.org/doc/essays/list2str/ -- http://mail.python.org/mailman/listinfo/python-list