Fastest way to get a hash from a list?

Question

I have a long list of integers that I want to turn into an MD5 hash. What's the quickest way to do this? I have tried two options, both similar. Just wondering if I'm missing an obviously quicker method.

import random
import hashlib
import cPickle as pickle

r = [random.randrange(1, 1000) for _ in range(0, 1000000)]

def method1(r):
    p = pickle.dumps(r, -1)
    return hashlib.md5(p).hexdigest()

def method2(r):
    p = str(r)
    return hashlib.md5(p).hexdigest()

def method3(r):
    p = ','.join(map(str, r))
    return hashlib.md5(p).hexdigest()
    

Then time it in iPython:

timeit method1(r)
timeit method2(r)
timeit method3(r)

Gives me this:

In [8]: timeit method1(r)
10 loops, best of 3: 68.7 ms per loop

In [9]: timeit method2(r)
10 loops, best of 3: 176 ms per loop

In [10]: timeit method3(r)
1 loops, best of 3: 270 ms per loop

So, option 1 is the best I've got. But I have to do it a lot and it's currently the rate determining step in my code.

Any tips or tricks to get a unique hash from a big list quicker than what's here, using Python 2.7?

Answer 1

You may find this useful. It uses my own custom bench-marking framework (based on timeit) to gather and print the results. Since the variations in speed are primarily due to the need to convert the r list to something that hashlib.md5() can work with, I've updated the suite of test cases to show how storing the values in an array.array instead, as @DSM suggested in a comment, would dramatically speed things up. Note that since the integers in the list are all relatively small I've stored them in an array of short (2-byte) values.

from __future__ import print_function
import sys
import timeit

setup = """
import array
import random
import hashlib
import marshal
import cPickle as pickle
import struct

r = [random.randrange(1, 1000) for _ in range(0, 1000000)]
ra = array.array('h', r)   # create an array of shorts equivalent

def method1(r):
    p = pickle.dumps(r, -1)
    return hashlib.md5(p).hexdigest()

def method2(r):
    p = str(r)
    return hashlib.md5(p).hexdigest()

def method3(r):
    p = ','.join(map(str, r))
    return hashlib.md5(p).hexdigest()

def method4(r):
    fmt = '%dh' % len(r)
    buf = struct.pack(fmt, *r)
    return hashlib.md5(buf).hexdigest()

def method5(r):
    a = array.array('h', r)
    return hashlib.md5(a).hexdigest()

def method6(r):
    m = marshal.dumps(r)
    return hashlib.md5(m).hexdigest()

# using pre-built array...
def pb_method1(ra):
    p = pickle.dumps(ra, -1)
    return hashlib.md5(p).hexdigest()

def pb_method2(ra):
    p = str(ra)
    return hashlib.md5(p).hexdigest()

def pb_method3(ra):
    p = ','.join(map(str, ra))
    return hashlib.md5(p).hexdigest()

def pb_method4(ra):
    fmt = '%dh' % len(ra)
    buf = struct.pack(fmt, *ra)
    return hashlib.md5(buf).hexdigest()

def pb_method5(ra):
    return hashlib.md5(ra).hexdigest()

def pb_method6(ra):
    m = marshal.dumps(ra)
    return hashlib.md5(m).hexdigest()
"""

statements = {
    "pickle.dumps(r, -1)": """
        method1(r)
    """,
    "str(r)": """
        method2(r)
    """,
    "','.join(map(str, r))": """
        method3(r)
    """,
    "struct.pack(fmt, *r)": """
        method4(r)
    """,
    "array.array('h', r)": """
        method5(r)
    """,
    "marshal.dumps(r)": """
        method6(r)
    """,
# versions using pre-built array...
    "pickle.dumps(ra, -1)": """
        pb_method1(ra)
    """,
    "str(ra)": """
        pb_method2(ra)
    """,
    "','.join(map(str, ra))": """
        pb_method3(ra)
    """,
    "struct.pack(fmt, *ra)": """
        pb_method4(ra)
    """,
    "ra (pre-built)": """
        pb_method5(ra)
    """,
    "marshal.dumps(ra)": """
        pb_method6(ra)
    """,
}

N = 10
R = 3

timings = [(
    idea,
    min(timeit.repeat(statements[idea], setup=setup, repeat=R, number=N)),
    ) for idea in statements]

longest = max(len(t[0]) for t in timings)  # length of longest name

print('fastest to slowest timings (Python {}.{}.{})\n'.format(*sys.version_info[:3]),
      '  ({:,d} calls, best of {:d})\n'.format(N, R))

ranked = sorted(timings, key=lambda t: t[1])  # sort by speed (fastest first)
for timing in ranked:
    print("{:>{width}} : {:.6f} secs, rel speed {rel:>8.6f}x".format(
          timing[0], timing[1], rel=timing[1]/ranked[0][1], width=longest))

Results:

fastest to slowest timings (Python 2.7.6)
   (10 calls, best of 3)

        ra (pre-built) : 0.037906 secs, rel speed 1.000000x
     marshal.dumps(ra) : 0.177953 secs, rel speed 4.694626x
      marshal.dumps(r) : 0.695606 secs, rel speed 18.350932x
   pickle.dumps(r, -1) : 1.266096 secs, rel speed 33.401179x
   array.array('h', r) : 1.287884 secs, rel speed 33.975950x
  pickle.dumps(ra, -1) : 1.955048 secs, rel speed 51.576558x
  struct.pack(fmt, *r) : 2.085602 secs, rel speed 55.020743x
 struct.pack(fmt, *ra) : 2.357887 secs, rel speed 62.203962x
                str(r) : 2.918623 secs, rel speed 76.996860x
               str(ra) : 3.686666 secs, rel speed 97.258777x
 ','.join(map(str, r)) : 4.701531 secs, rel speed 124.032173x
','.join(map(str, ra)) : 4.968734 secs, rel speed 131.081303x

Answer 2

You can improve performance slightly, simplify your code, and remove an import by using Python's builtin hash function instead of md5 from hashlib:

import random
import cPickle as pickle

r = [random.randrange(1, 1000) for _ in range(0, 1000000)]

def method1(r):
    p = pickle.dumps(r, -1)
    return hash(p)

def method2(r):
    p = str(r)
    return hash(p)

def method3(r):
    p = ','.join(map(str, r))
    return hash(p)