tags: python struct
title: Python Struct Pack Unpack

See [the docs](https://docs.python.org/3/library/struct.html).

## Examples
Pseudorandomly and deterministically choose from a list of arguments.
The first argument `idx` is hashed, turned into an integer, reduced modulo
the length of the list of remaining arguments, and that argument returned.
That is, given the same `idx`, and the same list of arguments, it will 
return the same argument every time (unlike using `random.choice`).
```py
import sys, hashlib, struct
args = sys.argv[1:]
if len(args) < 2:
  print(f"choosed idx args...",file=sys.stderr)
  print(f"  idx is hashed and reduced mod to choose")
  exit(1)
idx, *args = args
sha = hashlib.sha256()
sha.update(idx.encode())
h = sha.digest()
i, = struct.unpack(">Q",h[:8])
print(args[i%len(args)])
```