Empty OPENAI_BASE_URL prevents fallback to default API endpoint

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

I noticed a small issue in the base_url initialization logic: (https://github.com/openai/openai-python/blob/656e3cab4a18262a49b961d41293367e45ee71b9/src/openai/_client.py#L163-L166)

if base_url is None:
    base_url = os.environ.get("OPENAI_BASE_URL")
if base_url is None:
    base_url = f"https://api.openai.com/v1"

If the environment variable OPENAI_BASE_URL is defined but empty (e.g., OPENAI_BASE_URL=""), then os.environ.get("OPENAI_BASE_URL") returns an empty string. As a result:

• base_url becomes ""
• the second condition (if base_url is None) is never triggered
• the intended fallback "https://api.openai.com/v1" is skipped

An empty string is not a valid base URL, so this can lead to confusing behavior.

To Reproduce

1. export OPENAI_BASE_URL=""
2. run the following:

from openai import OpenAI

client = OpenAI()
client.models.list()

3. Trigger an "openai.APIConnectionError: Connection error" error

Code snippets

OS

macOS

Python version

Python 3.12.2

Library version

2.15.0

Start in src/openai/_client.py at the base_url initialization logic linked in the issue, then reproduce the behavior with OPENAI_BASE_URL set to an empty string and client.models.list(). Done means the client uses the documented default endpoint instead of producing a connection error in this environment.