I'm not sure where (or if) it's defined in the Python docs, but in PEP 3101 it's in https://www.python.org/dev/peps/pep-3101/#simple-and-compound-field-names: "It should be noted that the use of 'getitem' within a format string is much more limited than its conventional usage. In the above example, the string 'name' really is the literal string 'name', not a variable named 'name'. The rules for parsing an item key are very simple. If it starts with a digit, then it is treated as a number, otherwise it is used as a string.".