Inconsistent base_url behavior with module client and OpenAIClient

Confirm this is an issue with the Python library and not an underlying OpenAI API

• This is an issue with the Python library

Describe the bug

The issue is that the base_url in the OpenAI client enforces a trailing slash whereas setting the module openai.base_url does not enforce.

This is 100% reproducible and while it can be worked around, the inconsistency in clients causes issues in scripts that use both clients.

To Reproduce

1. Create a module client and use a base_url without a trailing slash. See error with URL for completions
2. Create an OpenAIClient and use a base_url without a trailing slash. See no erros.

Code snippets

This works

    client = openai.OpenAI(
        api_key=my_key,
        base_url="https://myllmserver.com/api/v1"
    )
    print(client.completions.create(
       model="mistralai/Mistral-7B-Instruct-v0.2",
       prompt="test" 
    ))

This does not:

    openai.base_url="https://myllmserver.com/api/v1"
    openai.api_key=my_key
    print(openai.completions.create(
       model="mistralai/Mistral-7B-Instruct-v0.2",
       prompt="test"
    ))

Error:

https://myllmserver.com/api/v1completions "HTTP/1.1 404 Not Found"

OS

macOS

Python version

Python v3.12

Library version

openai v1.23.6

Start by tracing how base_url is handled for the module client and OpenAI client, using the two reproduction snippets as the behavioral comparison. Make both clients construct the same valid completion URL when the base URL lacks a trailing slash, then verify the two call paths produce consistent requests.