Harrison/token counts (#311)

Co-authored-by: thepok <richterthepok@yahoo.de>

langchain/llms/openai.py

@@ -116,9 +116,86 @@ class OpenAI(LLM, BaseModel):
                 response = openai("Tell me a joke.")
         """
         params = self._default_params
+
+        if params["max_tokens"] == -1:
+            params["max_tokens"] = self.max_tokens_for_prompt(prompt)
+
         if stop is not None:
             if "stop" in params:
                 raise ValueError("`stop` found in both the input and default params.")
             params["stop"] = stop
         response = self.client.create(model=self.model_name, prompt=prompt, **params)
         return response["choices"][0]["text"]
+
+    def modelname_to_contextsize(self, modelname: str) -> int:
+        """Calculate the maximum number of tokens possible to generate for a model.
+
+        text-davinci-003: 4,000 tokens
+        text-curie-001: 2,048 tokens
+        text-babbage-001: 2,048 tokens
+        text-ada-001: 2,048 tokens
+        code-davinci-002: 8,000 tokens
+        code-cushman-001: 2,048 tokens
+
+        Args:
+            modelname: The modelname we want to know the context size for.
+
+        Returns:
+            The maximum context size
+
+        Example:
+            .. code-block:: python
+
+                max_tokens = openai.modelname_to_contextsize("text-davinci-003")
+        """
+        if modelname == "text-davinci-003":
+            return 4000
+        elif modelname == "text-curie-001":
+            return 2048
+        elif modelname == "text-babbage-001":
+            return 2048
+        elif modelname == "text-ada-001":
+            return 2048
+        elif modelname == "code-davinci-002":
+            return 8000
+        elif modelname == "code-cushman-001":
+            return 2048
+        else:
+            return 4000
+
+    def max_tokens_for_prompt(self, prompt: str) -> int:
+        """Calculate the maximum number of tokens possible to generate for a prompt.
+
+        Args:
+            prompt: The prompt to pass into the model.
+
+        Returns:
+            The maximum number of tokens to generate for a prompt.
+
+        Example:
+            .. code-block:: python
+
+                max_tokens = openai.max_token_for_prompt("Tell me a joke.")
+        """
+        # TODO: this method may not be exact.
+        # TODO: this method may differ based on model (eg codex).
+        try:
+            from transformers import GPT2TokenizerFast
+        except ImportError:
+            raise ValueError(
+                "Could not import transformers python package. "
+                "This is needed in order to calculate max_tokens_for_prompt. "
+                "Please it install it with `pip install transformers`."
+            )
+        # create a GPT-3 tokenizer instance
+        tokenizer = GPT2TokenizerFast.from_pretrained("gpt2")
+
+        # tokenize the text using the GPT-3 tokenizer
+        tokenized_text = tokenizer.tokenize(prompt)
+
+        # calculate the number of tokens in the tokenized text
+        num_tokens = len(tokenized_text)
+
+        # get max context size for model by name
+        max_size = self.modelname_to_contextsize(self.model_name)
+        return max_size - num_tokens