Note a few implementation details or restrictions.

pep-0498.txt

@@ -580,6 +580,29 @@ python-config.py::
   print("Usage: {0} [{1}]".format(sys.argv[0], '|'.join('--'+opt for opt in valid_opts)), file=sys.stderr)
   print(f"Usage: {sys.argv[0]} [{'|'.join('--'+opt for opt in valid_opts)}]", file=sys.stderr)
 
+Implementation limitations
+==========================
+
+Maximum of 255 expressions
+--------------------------
+
+Due to a CPython limit with the number of parameters to a function, an
+f-string may not contain more that 255 expressions. This includes
+expressions inside format specifiers. So this code would count as
+having 2 expressions::
+
+  f'{x:.{width}}'
+
+Expressions with side effects
+-----------------------------
+
+xxx
+
+Expressions used multiple times
+-------------------------------
+
+xxx
+
 References
 ==========