Raise an error when a placeholder is used in an f-string or .format() call.

This is a common mistake that can lead to confusing errors, because when combined with surrounding strings, it can lead to invalid values like `--data_path=input:<tfx.dsl.placeholder.placeholder.ArtifactPlaceholder\ object\ at\ 0x7f42d66671f0>/filename.txt`. The component implementation might receive as an argument in a place where it normally expects a proper file path, causing it to print weird errors or even read an empty file instead, depending on its implementation.

Note: If you actually intend to print the placeholder to the developer/console, use `repr()` instead.
PiperOrigin-RevId: 640889882

Author: tfx-copybara

RELEASE.md

@@ -4,6 +4,12 @@
 
 ## Breaking Changes
 
+*  `Placeholder.__format__()` is now disallowed, so you cannot use placeholders
+   in f-strings and `str.format()` calls anymore. If you get an error from this,
+   most likely you discovered a bug and should not use an f-string in the first
+   place. If it is truly your intention to print the placeholder (not its
+   resolved value) for debugging purposes, use `repr()` or `!r` instead.
+
 ### For Pipeline Authors
 
 ### For Component Authors

tfx/dsl/experimental/conditionals/conditional.py

@@ -36,7 +36,7 @@ def validate(self, containing_nodes: Sequence[base_node.BaseNode]):
         if id(ancestor_context.predicate) == id(self.predicate):
           raise ValueError(
               'Nested conditionals with duplicate predicates:\n'
-              f'{self.predicate} vs\n{ancestor_context.predicate}.\n'
+              f'{self.predicate!r} vs\n{ancestor_context.predicate!r}.\n'
               'Please merge the redundant conditionals.'
           )
 

tfx/dsl/placeholder/placeholder_base.py

@@ -145,6 +145,14 @@ def __iter__(self) -> Iterator[Any]:
         'Did you miss the ending `,` in your tuple?'
     )
 
+  def __format__(self, format_spec) -> str:
+    raise RuntimeError(
+        'Formatting a placeholder is not supported. Did you accidentally use a '
+        'placeholder inside an f-string or .format() call? That cannot work '
+        'because placeholder values are only known later at runtime. You can '
+        'use the + operator for string concatenation.'
+    )
+
   def b64encode(self, url_safe: bool = True) -> _Base64EncodeOperator:
     """Encodes the value with URL-safe Base64 encoding."""
     return _Base64EncodeOperator(self, url_safe)

tfx/types/channel_utils.py

@@ -248,7 +248,7 @@ def unwrap_simple_channel_placeholder(
   ):
     raise ValueError(
         'Expected placeholder of shape somechannel.future()[0].value, but got'
-        f' {placeholder}.'
+        f' {placeholder!r}.'
     )
 
   # Now that we know there's only one channel inside, we can just extract it: