issue #904. Err when no props in metadata (#905)

Co-authored-by: Martin Durant <[email protected]>

fastparquet/util.py

@@ -297,14 +297,18 @@ def update_custom_metadata(obj, custom_metadata : dict):
         Thrift object or parquet file which metadata is to update.
     custom_metadata : dict
         Key-value metadata to update in thrift object.
-        
+        The values must be strings or binary. To pass a dictionary, serialize it as json string then encode it in binary.
     Notes
     -----
     Key-value metadata are expected binary encoded. This function ensures it
     is.
     """
     kvm = (obj.key_value_metadata if isinstance(obj, ThriftObject)
            else obj.fmd.key_value_metadata)
+
+    if kvm is None:
+        kvm = []
+
     # Spare list of keys.
     kvm_keys = [item.key for item in kvm]
     for key, value in custom_metadata.items():

fastparquet/writer.py

@@ -1611,6 +1611,7 @@ def update_file_custom_metadata(path: str, custom_metadata: dict,
         Local path to file.
     custom_metadata : dict
         Key-value metadata to update in thrift object.
+        The values must be strings or binary. To pass a dictionary, serialize it as json string then encode it in binary.
     is_metadata_file : bool, default None
         Define if target file is a pure metadata file, or is a parquet data
         file. If `None`, is set depending file name.