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commit 63f7ec889d71ab7a65a19ae6b1efd404eb30353d
Author: Charles Chen <>
AuthorDate: Mon Feb 5 16:28:03 2018 -0800

    Clarify runtime type checking documentation
 src/documentation/sdks/ | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/documentation/sdks/ 
index ae8fc09..3962fe1 100644
--- a/src/documentation/sdks/
+++ b/src/documentation/sdks/
@@ -106,16 +106,18 @@ The following are special type hints that don't 
correspond to a class, but rathe
 In addition to using type hints for type checking at pipeline construction, 
you can enable runtime type checking to check that actual elements satisfy the 
declared type constraints during pipeline execution.
-For example, the following code would pass at both pipeline construction and 
+For example, the following pipeline emits elements of the wrong type. 
Depending on the runner implementation, its execution may or may not fail at 
 {% github_sample 
 tag:type_hints_runtime_off %}```
-However, if you enable runtime type checking, the code passes at pipeline 
construction and fails at runtime. To enable runtime type checking, set the 
pipeline option `runtime_type_check` to `True`.
+However, if you enable runtime type checking, the code is guaranteed to fail 
at runtime. To enable runtime type checking, set the pipeline option 
`runtime_type_check` to `True`.
 {% github_sample 
 tag:type_hints_runtime_on %}```
+Note that because runtime type checks are done for each `PCollection` element, 
enabling this feature may incur a significant performance penalty. It is 
therefore recommended that runtime type checks are disabled for production 
 ## Use of Type Hints in Coders
 When your pipeline reads, writes, or otherwise materializes its data, the 
elements in your `PCollection` need to be encoded and decoded to and from byte 
strings. Byte strings are used for intermediate storage, for comparing keys in 
`GroupByKey` operations, and for reading from sources and writing to sinks.

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