How to read lines from a mmapped file?

Question

Is seems that the mmap interface only supports readline(). If I try to iterate over the object I get character instead of complete lines.

What would be the "pythonic" method of reading a mmap'ed file line by line?

import sys
import mmap
import os


if (len(sys.argv) > 1):
  STAT_FILE=sys.argv[1]
  print STAT_FILE
else:
  print "Need to know <statistics file name path>"
  sys.exit(1)


with open(STAT_FILE, "r") as f:
  map = mmap.mmap(f.fileno(), 0, prot=mmap.PROT_READ)
  for line in map:
    print line # RETURNS single characters instead of whole line

Answer 1

The most concise way to iterate over the lines of an mmap is

with open(STAT_FILE, "r+b") as f:
    map_file = mmap.mmap(f.fileno(), 0, prot=mmap.PROT_READ)
    for line in iter(map_file.readline, b""):
        # whatever

Note that in Python 3 the sentinel parameter of iter() must be of type bytes, while in Python 2 it needs to be a str (i.e. "" instead of b"").

Answer 2

I modified your example like this:

with open(STAT_FILE, "r+b") as f:
        m=mmap.mmap(f.fileno(), 0, prot=mmap.PROT_READ)
        while True:
                line=m.readline()
                if line == '': break
                print line.rstrip()

Suggestions:

• Do not call a variable map, this is a built-in function.
• Open the file in r+b mode, as in the Python example on the mmap help page. It states: In either case you must provide a file descriptor for a file opened for update. See http://docs.python.org/library/mmap.html#mmap.mmap.
• It's better to not use UPPER_CASE_WITH_UNDERSCORES global variable names, as mentioned in Global Variable Names at https://www.python.org/dev/peps/pep-0008/#global-variable-names. In other programming languages (like C), constants are often written all uppercase.

Hope this helps.

Edit: I did some timing tests on Linux because the comment made me curious. Here is a comparison of timings made on 5 sequential runs on a 137MB text file.

Normal file access:

real    2.410 2.414 2.428 2.478 2.490
sys     0.052 0.052 0.064 0.080 0.152
user    2.232 2.276 2.292 2.304 2.320

mmap file access:

real    1.885 1.899 1.925 1.940 1.954
sys     0.088 0.108 0.108 0.116 0.120
user    1.696 1.732 1.736 1.744 1.752

Those timings do not include the print statement (I excluded it). Following these numbers I'd say memory mapped file access is quite a bit faster.

Edit 2: Using python -m cProfile test.py I got the following results:

5432833    2.273    0.000    2.273    0.000 {method 'readline' of 'file' objects}
5432833    1.451    0.000    1.451    0.000 {method 'readline' of 'mmap.mmap' objects}

If I'm not mistaken then mmap is quite a bit faster.

Additionally, it seems not len(line) performs worse than line == '', at least that's how I interpret the profiler output.

Answer 3

The following is reasonably concise:

with open(STAT_FILE, "r") as f:
    m = mmap.mmap(f.fileno(), 0, prot=mmap.PROT_READ)
    while True:
        line = m.readline()  
        if line == "": break
        print line
    m.close()

Note that line retains the newline, so you might like to remove it. It is also the reason why if line == "" does the right thing (an empty line is returned as "\n").

The reason the original iteration works the way it does is that mmap tries to look like both a file and a string. It looks like a string for the purposes of iteration.

I have no idea why it can't (or chooses not to) provide readlines()/xreadlines().