add loading chains from hub (#757)

langchain/chains/loading.py

@@ -1,15 +1,20 @@
 """Functionality for loading chains."""
 import json
+import os
+import tempfile
 from pathlib import Path
 from typing import Union
 
+import requests
 import yaml
 
 from langchain.chains.base import Chain
 from langchain.chains.llm import LLMChain
 from langchain.llms.loading import load_llm, load_llm_from_config
 from langchain.prompts.loading import load_prompt, load_prompt_from_config
 
+URL_BASE = "https://raw.githubusercontent.com/hwchase17/langchain-hub/master/chains/"
+
 
 def _load_llm_chain(config: dict) -> LLMChain:
     """Load LLM chain from config dict."""
@@ -48,7 +53,16 @@ def load_chain_from_config(config: dict) -> Chain:
     return chain_loader(config)
 
 
-def load_chain(file: Union[str, Path]) -> Chain:
+def load_chain(path: Union[str, Path]) -> Chain:
+    """Unified method for loading a chain from LangChainHub or local fs."""
+    if isinstance(path, str) and path.startswith("lc://chains"):
+        path = os.path.relpath(path, "lc://chains/")
+        return _load_from_hub(path)
+    else:
+        return _load_chain_from_file(path)
+
+
+def _load_chain_from_file(file: Union[str, Path]) -> Chain:
     """Load chain from file."""
     # Convert file to Path object.
     if isinstance(file, str):
@@ -66,3 +80,19 @@ def load_chain(file: Union[str, Path]) -> Chain:
         raise ValueError("File type must be json or yaml")
     # Load the chain from the config now.
     return load_chain_from_config(config)
+
+
+def _load_from_hub(path: str) -> Chain:
+    """Load chain from hub."""
+    suffix = path.split(".")[-1]
+    if suffix not in {"json", "yaml"}:
+        raise ValueError("Unsupported file type.")
+    full_url = URL_BASE + path
+    r = requests.get(full_url)
+    if r.status_code != 200:
+        raise ValueError(f"Could not find file at {full_url}")
+    with tempfile.TemporaryDirectory() as tmpdirname:
+        file = tmpdirname + "/chain." + suffix
+        with open(file, "wb") as f:
+            f.write(r.content)
+        return _load_chain_from_file(file)