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    <p>There is no function that does this, and your method is by far the fastest approach.</p> <p>I was going to suggest <code>bytearray(mmap)</code>, the <code>array.fromfile</code>, and even a homebrew <code>os.read()</code> using <code>bytearray</code> and <code>memoryview</code>, but <code>FileIO.readinto</code> is <em>screaming fast</em>.</p> <pre><code>import os import mmap, io, array import timeit fn = 'path_to_largish_file' def fd_readinto_mmap(fd, ba): m = mmap.mmap(fd, 0, access=mmap.ACCESS_READ) ba.extend(m) m.close() def fd_readinto_fio(fd, ba): with io.FileIO(fd, closefd = False) as fio: fio.readinto(ba) def fd_readinto_array(fd, ba): ar = array.array('c') sz = os.fstat(fd).st_size fp = os.fdopen(fd, 'rb') ar.fromfile(fp, sz) ba.extend(ar) def fd_readinto_mv(fd, ba): stat = os.fstat(fd) blksize = getattr(stat, 'st_blksize', 4096) bufsize = stat.st_size buf = bytearray(bufsize) m = memoryview(buf) while True: b = os.read(fd, blksize) s = len(b) if not s: break m[:s], m = b, m[s:] writtenbytes = buffer(buf, 0, bufsize-len(m)) ba.extend(writtenbytes) setup = """ from __main__ import fn, fd_readinto_mmap, fd_readinto_fio, fd_readinto_array, fd_readinto_mv, reset_fd import os openfd = lambda : os.open(fn, os.O_RDONLY) closefd = lambda fd: os.close(fd) """ reps = 2 tests = { 'fio' : "fd=openfd(); fd_readinto_fio(fd, bytearray()); closefd(fd)", 'mmap': "fd=openfd(); fd_readinto_mmap(fd, bytearray()); closefd(fd)", 'array': "fd=openfd(); fd_readinto_array(fd, bytearray());", 'mv' : "fd=openfd(); fd_readinto_mv(fd, bytearray()); closefd(fd)", } width = max(map(len, tests)) for n,t in tests.iteritems(): time = timeit.timeit(t, setup, number=reps) print ("{:%s} {}" % width).format(n, time) </code></pre> <p>On my system, <code>FileIO</code> is orders of magnitude faster. <code>FileIO.readinto</code> seems to be nearly constant-time with respect to file size, too. (Not sure how that is even possible.)</p> <pre><code>mmap 4.86922478676 array 4.19783091545 mv 7.75051403046 fio 9.29832458496e-05 </code></pre>
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    1. POpython read from fd directly into bytearray
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    1. USFrancis Avila
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    1. POpython read from fd directly into bytearray
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    1. COI hate to say this -- but your analysis is flawed. According to these results, fio is ~100,000 times faster than everything else (which caused my left eyebrow to raise quite high <grin>). I spent some time examining your code, and discovered the flaw. According to the docs (http://docs.python.org/2.7/library/io.html#io.RawIOBase.readinto), "...readinto(b) reads up to len(b) bytes..." In your example, you are passing an empty bytearray(), so it returns zero bytes. If you pre-grow the bytearray() to it's appropriate size, you'll get saner results.
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