Why is sys.path a list?

Question

Why would the implementers choose to make sys.path into a list as opposed to a ordered set?

Having sys.path as a list gives rise to the possibility of having multiple duplicates in the path, slowing down the search time for modules.

An artificial example would be the following silly example

# instant importing
import os
import sys

for i in xrange(50000):
    sys.path.insert(0, os.path.abspath(".")

# importing takes a while to fail
import hello

To summarise from the comments and answers given:

It seems from the responses below that a list is a simple structure which handles 99% of everyone's needs, it does not come with a safety feature of avoiding duplicates however it does come with a primitive prioritisation which is the index of the element in the list where you can easily set the highest priority by prepending or lowest priority by appending.

Adding a richer prioritisation i.e. insert before this element would be rarely used as the interface to this would be too much effort for a simple task. As the accepted answer states, there is no practical need for anything more advanced covering these extra use cases as historically people are used to this.

Answer 1

• Ordered set is
  • a very recent idea (the recipe mentioned in Does Python have an ordered set? is for 2.6+)
  • a very special-purpose structure (not even in the standard library)
• There's no practical need for the added complexity
  • List is a very simple structure, while ordered set is basically a hash table + list + weaving logic
  • You don't need to do operations with sys.path that a set is designed for - check if the exact path is in sys.path - even less so, do it very quickly
  • On the contrary, sys.path's typical use cases are those exactly for a list: trying elements in sequence, prepending or appending one

To summarize, there's both a historical reason and a lack of any practical need.

Answer 2

sys.path specifies a search path. Typically search paths are ordered with the order of the items indicating search order. If sys.path was a set then there would be no explicit ordering making sys.path less useful. It's also worth considering that optimization is a tricky issue. A reasonable optimization to address any performance concerns would be to simply keep a record of already searched elements of sys.path. Trying to be tricky with ordered sets probably isn't worth the effort.