[GitHub] [arrow] nealrichardson commented on a change in pull request #10326: ARROW-12791: [R] Better error handling for DatasetFactory$Finish() when no format specified

[GitBox <gi...@apache.org>]

nealrichardson commented on a change in pull request #10326:
URL: https://github.com/apache/arrow/pull/10326#discussion_r634597442



##########
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:
       The error isn't wrong, it's just not informative at this higher layer. "Invalid parquet magic bytes" is meaningful if you yourself called read_parquet(file), but in this case, you weren't intending to read a parquet file because they aren't parquet files. 

##########
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:
       The error isn't wrong, it's just not informative at this particular higher layer. "Invalid parquet magic bytes" is meaningful if you yourself called read_parquet(file), but in this case, you weren't intending to read a parquet file because they aren't parquet files. 




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