SSL error downloading NLTK data

You don't need to disable SSL checking if you run the following terminal command:

/Applications/Python 3.6/Install Certificates.command

In the place of 3.6, put your version of Python if it's an earlier one. Then you should be able to open your Python interpreter (using the command python3) and successfully run nltk.download() there.

This is an issue wherein urllib uses an embedded version of OpenSSL that not in the system certificate store. Here's an answer with more information on what's going on.

Please see answer by @doctorBroctor. It is more correct and safer to use. Leaving answer below as it might be useful for something else.

https://stackoverflow.com/a/42890688/1167890

---

This will work by disabling SSL checking.

import nltk
import ssl

try:
    _create_unverified_https_context = ssl._create_unverified_context
except AttributeError:
    pass
else:
    ssl._create_default_https_context = _create_unverified_https_context

nltk.download()