How I make cache-keys from python objects
=========================================

When I need a consistent key for a pythohn object I often reach for It works for me and the use cases I have.

Date: March 30, 2022

When I need a consistent key for a pythohn object I often reach for
`hashlib.md5` It works for me and the use cases I have.

## diskcache

Yesterday we talked about setting up a persistant cache with python diskcache.
In order to make this really work we need a good way to make consistent cache
keys from some sort of python object.

How I setup a sqlite cache in python [1]

## hash

_does not work_

My first thought was to just hash the files, this will give me a unique key for
each. This will work, and give you a consistant key for one and only one given
python process. If you start a new interpreter you will get different keys.

```python
waylonwalker.com on  main [$✘!?] via  v5.1.5  v3.8.0 (waylonwalker.com)
❯ ipython

waylonwalker ↪main v3.8.0 ipython
❯ hash("waylonwalker")
-3862245013515310359

waylonwalker ↪main v3.8.0 ipython
❯ hash("waylonwalker")
-3862245013515310359

waylonwalker ↪main v3.8.0 ipython
❯ exit

waylonwalker.com on  main [$✘!?] via  v5.1.5  v3.8.0 (waylonwalker.com)
❯ ipython

waylonwalker ↪main v3.8.0 ipython
❯ hash("waylonwalker")
-83673051278873734

```

> here is a snapshot of my terminal proving that you can get the same hash in one session, but it changes when you restart ipython.

## hashlib.md5

Here is a quick couple ipython sessions showing that md5 cache is consistent accross multiple sessions.

```python
waylonwalker.com on  main [$✘!?] via  v5.1.5  v3.8.0 (waylonwalker.com) on  (us-east-1)
❯ ipython

waylonwalker ↪main v3.8.0 ipython
❯ hashlib.md5("waylonwalker")
[PYFLYBY] import hashlib
╭─────────────────────────────── Traceback (most recent call last) ────────────────────────────────╮
│ :1 in │
╰──────────────────────────────────────────────────────────────────────────────────────────────────╯
TypeError: Unicode-objects must be encoded before hashing

waylonwalker ↪main v3.8.0 ipython
❯ hashlib.md5("waylonwalker".encode("utf-8"))

waylonwalker ↪main v3.8.0 ipython
❯ hashlib.md5("waylonwalker".encode("utf-8")).hexdigest()
'1c7c1073ca096ffdb324471770911fe2'

waylonwalker ↪main v3.8.0 ipython
❯ hashlib.md5("waylonwalker".encode("utf-8")).hexdigest()
'1c7c1073ca096ffdb324471770911fe2'

waylonwalker ↪main v3.8.0 ipython
❯ hashlib.md5("waylonwalker".encode("utf-8")).hexdigest()
'1c7c1073ca096ffdb324471770911fe2'

waylonwalker ↪main v3.8.0 ipython
❯ exit

waylonwalker.com on  main [$✘!?] via  v5.1.5  v3.8.0 (waylonwalker.com) on  (us-east-1) took 47s
❯ ipython

waylonwalker ↪main v3.8.0 ipython
❯ hashlib.md5("waylonwalker".encode("utf-8")).hexdigest()
[PYFLYBY] import hashlib
'1c7c1073ca096ffdb324471770911fe2'

```

## key for diskcache

Since it is consistent we can use it as a cache key for diskcache operations.
I setup a little funciton that allows me to pass a bunch of differnt things in
to cache. As long as the __str__ method exists and is gives the data that you
want to cache key on, this will work.

```python
def make_hash(self, *keys: str) -> str:
 str_keys = [str(key) for key in keys]
 return hashlib.md5("".join(str_keys).encode("utf-8")).hexdigest()
```

understanding python \*args and \*\*kwargs [2]

> If the *args is confusing, I have a full article on `*args` and `**kwargs`.

## See it in action

Here you can see it in action. Anything passed into the function gets to be
part of the key.

```
waylonwalker ↪main v3.8.0 ipython
❯ def make_hash(self, *keys: str) -> str:
...: str_keys = [str(key) for key in keys]
...: return hashlib.md5("".join(str_keys).encode("utf-8")).hexdigest()
...:

waylonwalker ↪main v3.8.0 ipython
❯ make_hash(1, "one", "1", 1.0)
'73901d019df012a1cdab826ce301217d'

waylonwalker ↪main v3.8.0 ipython
❯ exit

waylonwalker.com on  main [$✘!?] via  v5.1.5  v3.8.0 (waylonwalker.com) on  (us-east-1) took 19m19s
❯

waylonwalker.com on  main [$✘!?] via  v5.1.5  v3.8.0 (waylonwalker.com) on  (us-east-1)
❯ ipython

waylonwalker ↪main v3.8.0 ipython
❯ def make_hash(self, *keys: str) -> str:
...: str_keys = [str(key) for key in keys]
...: return hashlib.md5("".join(str_keys).encode("utf-8")).hexdigest()
[PYFLYBY] import hashlib

waylonwalker ↪main v3.8.0 ipython
❯ make_hash(1, "one", "1", 1.0)
'73901d019df012a1cdab826ce301217d'
```

References:
[1]: /python-diskcache/
[2]: /python-args-kwargs/