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dev/notes/custom_exceptions.md

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+Custom exceptions
+
+Create an `./exceptions.py`.
+
+Define any custom exceptions. Can append `Error` to the exception name if needed, otherwise be descriptive.
+
+E.g `MismatchedTokenError`, `ScopedPageAlreadyExists`.
+
+(If it would raise a `SystemExit(1)` use an Error).
+
+
+Can set custom attributes on the Exception:
+
+```
+class MismatchedTokenError(Exception):
+    def __init__(self, expected: int, given: int) -> None:
+        self.expected = expected
+        self.given = given
+        super().__init__()
+```
+
+Can refer to these when capturing the exception:
+
+```
+except exceptions.MismatchedTokenError as exec_info:
+    click.echo(exec_info.expected, exec_info.given)
+    # error handling
+```
+
+Alternatively you can pass arbitrary `*args` into the exceptions and capture them with `args[0]`:
+
+```
+raise exceptions.TemplateNotFoundError("some message")
+```
+
+```
+except exceptions.TemplateFileNotFoundError as exec_info:
+    cli_message(exec_info.args[0])
+```