Python hash() function on strings

Question

How does a hash value of some particular string is calculated in CPython2.7?

For instance, this code:

print hash('abcde' * 1000)

returns the same value even after I restart the Python process and try again (I did it many times).

So, it seems that id() (memory address) of the string doesn't used in this computation, right? Then how?

"returns the same value even after I restart the Python process and try again" - not guaranteed, and usually not true on Python 3. "it seems that id() (memory address) of the string doesn't used in this computation" - well, of course not. Otherwise, we wouldn't have the invariant that a == b implies hash(a) == hash(b). — user2357112
– user2357112, Commented Oct 28, 2016 at 4:30
I think you need to run help(hash) and help(id) to understand the difference between the two because they are not the same... — Tadhg McDonald-Jensen
– Tadhg McDonald-Jensen, Commented Oct 28, 2016 at 4:31
Maybe this thread will shed some light? stackoverflow.com/questions/6008026/… — Savir
– Savir, Commented Oct 28, 2016 at 4:32

Mateen Ulhaq · Accepted Answer · 2021-10-01 09:43:54Z

25

Hash values are not dependent on the memory location but the contents of the object itself. From the documentation:

Return the hash value of the object (if it has one). Hash values are integers. They are used to quickly compare dictionary keys during a dictionary lookup. Numeric values that compare equal have the same hash value (even if they are of different types, as is the case for 1 and 1.0).

See CPython's implementation of str.__hash__ in:

Objects/unicodeobject.c (for unicode_hash)
Python/pyhash.c (for _Py_HashBytes)

edited Oct 1, 2021 at 9:43

Mateen Ulhaq

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answered Oct 28, 2016 at 4:29

Selcuk

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4 Comments

d-d Over a year ago

thanks. so it seems, that the hash(string) will be the same from run to run, right?

Selcuk Over a year ago

@d-d No, it is not guaranteed to be the same every time, but it is guaranteed to return the same value within the same process. If you want a non-changing hash, use hashlib functions instead.

d-d Over a year ago

yeah, I thought about it, but I need something faster than any hash function in this module. may be murmur hash or so..

Selcuk Over a year ago

Check out various hash functions here

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