A string conversion is a condition list enclosed in reverse (or backward) quotes:
string_conversion: "`" condition_list "`"
A string conversion evaluates the contained condition list and converts the resulting object into a string according to rules specific to its type.
If the object is a string, a number, None, or a tuple, list or dictionary containing only objects whose type is one of these, the resulting string is a valid Python expression which can be passed to the built-in function eval() to yield an expression with the same value (or an approximation, if floating point numbers are involved).
(In particular, converting a string adds quotes around it and converts ``funny'' characters to escape sequences that are safe to print.)
It is illegal to attempt to convert recursive objects (e.g. lists or
dictionaries that contain a reference to themselves, directly or
indirectly.)