jonkeane commented on a change in pull request #10326:
URL: https://github.com/apache/arrow/pull/10326#discussion_r634528056



##########
File path: r/R/dataset.R
##########
@@ -93,8 +93,19 @@ open_dataset <- function(sources,
     return(dataset___UnionDataset__create(sources, schema))
   }
   factory <- DatasetFactory$create(sources, partitioning = partitioning, ...)
-  # Default is _not_ to inspect/unify schemas
-  factory$Finish(schema, isTRUE(unify_schemas))
+  
+  tryCatch(
+    # Default is _not_ to inspect/unify schemas
+    factory$Finish(schema, isTRUE(unify_schemas)),
+    error = function (e) {
+      msg <- conditionMessage(e)
+      if(grep("Parquet magic bytes not found in footer", msg)){
+        stop("Looks like these are not parquet files, did you mean to specify 
a 'format'?", call. = FALSE)

Review comment:
       I actually think "just" the message you have here is better than the 
status quo (and better than docs). But I think you could `paste` together the 
two messages and be good if you wanted to surface both?




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