Improve error message in st.experimental_memo when it returns an unev…

…aluated dataframe (streamlit#5515)

lib/streamlit/runtime/caching/cache_errors.py

@@ -24,7 +24,7 @@
 )
 
 
-def _get_cached_func_name_md(func) -> str:
+def get_cached_func_name_md(func) -> str:
     """Get markdown representation of the function name."""
     if hasattr(func, "__name__"):
         return "`%s()`" % func.__name__
@@ -36,7 +36,7 @@ def _get_cached_func_name_md(func) -> str:
 def get_return_value_type(return_value) -> str:
     if hasattr(return_value, "__module__") and hasattr(type(return_value), "__name__"):
         return f"`{return_value.__module__}.{type(return_value).__name__}`"
-    return _get_cached_func_name_md(return_value)
+    return get_cached_func_name_md(return_value)
 
 
 class CacheType(enum.Enum):
@@ -141,7 +141,7 @@ def __init__(
         cache_type: CacheType,
         cached_func: types.FunctionType,
     ):
-        func_name = _get_cached_func_name_md(cached_func)
+        func_name = get_cached_func_name_md(cached_func)
         decorator_name = (cache_type.value,)
 
         msg = (
@@ -165,7 +165,7 @@ def __init__(self, func: types.FunctionType, return_value: types.FunctionType):
         MarkdownFormattedException.__init__(
             self,
             f"""
-            Cannot serialize the return value (of type {get_return_value_type(return_value)}) in {_get_cached_func_name_md(func)}.  
+            Cannot serialize the return value (of type {get_return_value_type(return_value)}) in {get_cached_func_name_md(func)}.  
             `st.experimental_memo` uses [pickle](https://docs.python.org/3/library/pickle.html) to 
             serialize the function’s return value and safely store it in the cache without mutating the original object. Please convert the return value to a pickle-serializable type.  
             If you want to cache unserializable objects such as database connections or Tensorflow 

lib/streamlit/runtime/caching/cache_utils.py

@@ -38,9 +38,10 @@
 from google.protobuf.message import Message
 
 import streamlit as st
-from streamlit import util
+from streamlit import type_util, util
 from streamlit.elements import NONWIDGET_ELEMENTS
 from streamlit.elements.spinner import spinner
+from streamlit.errors import StreamlitAPIException
 from streamlit.logger import get_logger
 from streamlit.proto.Block_pb2 import Block
 from streamlit.runtime.caching.cache_errors import (
@@ -51,6 +52,7 @@
     UnhashableParamError,
     UnhashableTypeError,
     UnserializableReturnValueError,
+    get_cached_func_name_md,
 )
 from streamlit.runtime.caching.hashing import update_hash
 
@@ -253,6 +255,21 @@ def get_or_create_cached_value():
                 try:
                     cache.write_result(value_key, return_value, messages)
                 except TypeError:
+                    if type_util.is_type(
+                        return_value, "snowflake.snowpark.dataframe.DataFrame"
+                    ):
+
+                        class UnevaluatedDataFrameError(StreamlitAPIException):
+                            def __init__(self):
+                                super().__init__(
+                                    self,
+                                    f"""
+                                    The function {get_cached_func_name_md(func)} is decorated with `st.experimental_memo` but it returns an unevaluated 
+                                    dataframe of type `snowflake.snowpark.DataFrame`. Please call `collect()` or `to_pandas()` on the 
+                                    dataframe before returning it, so `st.experimental_memo` can serialize and cache it.""",
+                                )
+
+                        raise UnevaluatedDataFrameError
                     raise UnserializableReturnValueError(
                         return_value=return_value, func=cached_func.func
                     )