Check for single prompt in __call__ method of the BaseLLM class

[mwinterde]

Ensuring that users pass a single prompt when calling a LLM

• This PR adds a check to the __call__ method of the BaseLLM class to ensure that it is called with a single prompt
• Raises a ValueError if users try to call a LLM with a list of prompt and instructs them to use the generate method instead

Why this could be useful

I stumbled across this by accident. I accidentally called the OpenAI LLM with a list of prompts instead of a single string and still got a result:

>>> from langchain.llms import OpenAI
>>> llm = OpenAI()
>>> llm(["Tell a joke"]*2)
"\n\nQ: Why don't scientists trust atoms?\nA: Because they make up everything!"

It might be better to catch such a scenario preventing unnecessary costs and irritation for the user.

Proposed behaviour

>>> from langchain.llms import OpenAI
>>> llm = OpenAI()
>>> llm(["Tell a joke"]*2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/marcus/Projects/langchain/langchain/llms/base.py", line 291, in __call__
    raise ValueError(
ValueError: Argument `prompt` is expected to be a single string, not a list. If you want to run the LLM on multiple prompts, use `generate` instead.

Before submitting

Who can review?

Community members can review the PR once tests pass. Tag maintainers/contributors who might be interested:

[eyurtsev]

Thanks for adding the run time check! What do you think about flipping the logic so it'll catch more more cases?

[eyurtsev]

It's common to flip the logic:

if not instance(prompt, str):

this will then catch a case when prompt is a tuple or a set

[mwinterde]

Hi @eyurtsev, agree, makes more sense that way 👍

[eyurtsev]

Suggested change

	"Argument `prompt` is expected to be a single string, not a list. If "
	f"Argument `prompt` is expected to be a string. Instead found {type(prompt)}. If "

[hwchase17]

lgtm! thanks for adding!