Add message templates.

README.md

@@ -83,7 +83,12 @@ Implementation notes:
   [`eval`](https://docs.python.org/3/library/functions.html#eval). It is advised to take a peek and
   learn about how `eval` works.
 
-### Example
+### Placeholders
+
+In addition to test cases, re-usable placeholders can be defined in a top level element names `placeholders`, pieces of 
+text which can be reused in any test cases within the YAML test file - see example below.
+
+### Examples
 
 #### 1. Inline type expectations
 
@@ -148,6 +153,22 @@ Implementation notes:
     reveal_type(a)  # NR: .*str.*
 ```
 
+#### 6. Placeholders
+
+```yaml
+- case: test_placeholder
+  main: |
+    a = 1
+    b = 'hello'
+
+    reveal_type(a)  # N: {{ reveal }} "builtins.int"
+    reveal_type(b)  # NR: {{ reveal }} {{ str }}
+
+- placeholders:
+    reveal: "Revealed type is"
+    str: ".*str.*"
+```
+
 ## Options
 
 ```

pytest_mypy_plugins/collect.py

@@ -2,6 +2,7 @@
 import platform
 import sys
 import tempfile
+from collections import ChainMap
 from typing import TYPE_CHECKING, Any, Dict, Iterator, List, Mapping, Optional
 
 import pytest
@@ -83,7 +84,10 @@ def collect(self) -> Iterator["YamlTestItem"]:
         if not isinstance(parsed_file, list):
             raise ValueError(f"Test file has to be YAML list, got {type(parsed_file)!r}.")
 
-        for raw_test in parsed_file:
+        templates = ChainMap(*[t["placeholders"] for t in parsed_file if "placeholders" in t])
+
+        raw_tests = [c for c in parsed_file if "case" in c]
+        for raw_test in raw_tests:
             test_name_prefix = raw_test["case"]
             if " " in test_name_prefix:
                 raise ValueError(f"Invalid test name {test_name_prefix!r}, only '[a-zA-Z0-9_]' is allowed.")
@@ -98,7 +102,7 @@ def collect(self) -> Iterator["YamlTestItem"]:
                     test_name_suffix = ""
 
                 test_name = f"{test_name_prefix}{test_name_suffix}"
-                main_content = utils.render_template(template=raw_test["main"], data=params)
+                main_content = utils.render_template(template=raw_test["main"], data=ChainMap(params, templates))
                 main_file = File(path="main.py", content=main_content)
                 test_files = [main_file] + parse_test_files(raw_test.get("files", []))
                 expect_fail = raw_test.get("expect_fail", False)

pytest_mypy_plugins/tests/test-placeholders.yml

@@ -0,0 +1,19 @@
+- case: test_placeholder
+  main: |
+    a = 1
+    b = 'hello'
+
+    reveal_type(a)  # N: {{ reveal }} "builtins.int"
+    reveal_type(b)  # NR: {{ reveal }} {{ str }}
+
+- case: test_using_parametrized_for_templates
+  parametrized:
+     - rtype: "Revealed type is"
+  main: |
+    a = 1
+
+    reveal_type(a)  # N: {{ rtype }} "builtins.int"
+
+- placeholders:
+    reveal: "Revealed type is"
+    str: ".*str.*"