Assign strings to IDs in Python

Question

I am reading a text file with python, formatted where the values in each column may be numeric or strings.

When those values are strings, I need to assign a unique ID of that string (unique across all the strings under the same column; the same ID must be assigned if the same string appears elsewhere under the same column).

What would be an efficient way to do it?

Answer 1

Use a defaultdict with a default value factory that generates new ids:

ids = collections.defaultdict(itertools.count().next)
ids['a']  # 0
ids['b']  # 1
ids['a']  # 0

When you look up a key in a defaultdict, if it's not already present, the defaultdict calls a user-provided default value factory to get the value and stores it before returning it.

collections.count() creates an iterator that counts up from 0, so collections.count().next is a bound method that produces a new integer whenever you call it.

Combined, these tools produce a dict that returns a new integer whenever you look up something you've never looked up before.