Each of these implementations varies in some way from the language as documented in this manual, or introduces specific information beyond what’s covered in the standard Python documentation. Please refer to the implementation- specific documentation to determine what else you need to know about the specific implementation you’re using. The descriptions of lexical analysis and syntax use a modified BNF grammar notation. This uses the following style of definition: The first line says that a$_{name}$is an$_{lc_letter}$ followed by a sequence of zero or more$_{lc_letter}$ s and underscores. An lc_letter in turn is any of the single characters 'a' through 'z' . (This rule is actually adhered to for the names defined in lexical and grammar rules in this document.) Each rule begins with a name (which is the name defined by the rule) and ::= . A vertical bar ( | ) is used to separate alternatives; it is the least binding operator in this notation. A star ($_{*}$ ) means zero or more repetitions of the preceding item; likewise, a plus ( + ) means one or more repetitions, and a phrase enclosed in square brackets ( [ ] ) means zero or one occurrences (in other words, the enclosed phrase is optional). The$_{*}$and$_{+}$operators bind as tightly as possible; parentheses are used for grouping. Literal strings are enclosed in quotes. White space is only meaningful to separate tokens. Rules are normally contained on a single line; rules with many alternatives may be formatted alternatively with each line after the first beginning with a vertical bar. In lexical definitions (as the example above), two more conventions are used: Two literal characters separated by three dots mean a choice of any single character in the given (inclusive) range of ASCII characters. A phrase between angular brackets ($_{<...>}$ ) gives an informal description of the symbol defined; e.g., this could be used to describe the notion of ‘control character’ if needed. Even though the notation used is almost the same, there is a big difference between the meaning of lexical and syntactic definitions: a lexical definition operates on the individual characters of the input source, while a syntax definition operates on the stream of tokens generated by the lexical analysis. All uses of BNF in the next chapter (“Lexical Analysis”) are lexical definitions; uses in subsequent chapters are syntactic definitions. The Python Language Reference, Release 3.9.5 name ::= lc_letter ( lc_letter | "_")* lc_letter ::= "a"..."z" Chapter 1. Introduction 4 1.2 Notation