Python Language

Python language is similar to visual basic. Python script is also interpretered by an interpreter line by line, but there are many differences between the languages.

Python Rule Notation

The rule notation used in Python:

A rule begins with a ~~name~~ and ~~::=~~, and end with details.
A vertical bar | is used to separate alternatives.
A star* means zero or more repetitions of the preceding item item.
A plus + means one or more repetitions.
A optional phrase enclosed by square brackets [] means zero or one occurrences.
The grouped phrase enclosed by parentheses () means grouping.
The literal string enclosed by quotes "" means literal strings
White space is only meaningful to separate tokens.
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.

Example

name::=lc_letter (lc_letter | "_")
    lc_letter::="a"..."z"

Python Language Grammar

The typical elements of Python Language of version 3.8.x.

Identifiers

Identifiers or names are the name of a Python thing. Identifiers are unlimited in length but case is significant. The valid characters within the ASCII range ~~U+0001...U+007F~~ for identifiers are same as Python 2.x, the uppercase and lowercase letters A through Z, the underscore _, and except for the first character, the digits 0 through 9. Other chararcters outside the ASCII range are also supported by Python 3.0.

identifier::=xid_start xid_continue*
    id_start::=<all characters in general categories Lu, Ll, Lt, Lm, Lo, Nl, the underscore, and characters with the Other_ID_Start property>
    id_continue::=<all characters in id_start, plus characters in the categories Mn, Mc, Nd, Pc and others with the Other_ID_Continue property>
    xid_start::=<all characters in id_start whose NFKC normalization is in "id_start xid_continue*">
    xid_continue::=<all characters in id_continue whose NFKC normalization is in "id_continue*">

The Unicode category codes mentioned above stand for:

Lu - uppercase letters
Ll - lowercase letters
Lt - titlecase letters
Lm - modifier letters
Lo - other letters
Nl - letter numbers
Mn - nonspacing marks
Mc - spacing combining marks
Nd - decimal numbers
Pc - connector punctuations
Other_ID_Start - explicit list of characters in PropList.txt to support backwards compatibility
Other_ID_Continue - likewise

Keywords

Some words are used as reserved words or keywords of the Python language that have special meaning and cannot be used as ordinary identifiers.

Reserved Keywords

keyword::="and" | "as" | "assert" | "async" | "await" | "break" | "class" | "continue" | "def" | "del" | "elif" | "else" | "except" | "False" | "finally" | "for" | "from" | "global" | "if" | "import" | "in" | "is" | "lambda" | "None" | "nonlocal" | "not" | "or" | "pass" | "raise" | "return" | "True" | "try" | "while" | "with" | "yield" |
"print" | "exec"

Reserved Classed of Identifiers

Some classes of identifiers with the patterns of leading and trailing underscore characters having special meanings are also reserved. _*Not import by from module import *. The sepcial identifier _ is used in the interactive interpreter to store the result of the last evaluation. When not in interactive mode, _ has no special meaning and is not defined. The name _ is often used in conjunction with internationalization __*__System-defined names, informally known as “dunder” names. These names are defined by the interpreter and its implementation (including the standard library). __*Class-private names. Names in this category, when used within the context of a class definition, are re-written to use a mangled form to help avoid name clashes between “private” attributes of base and derived classes.

Literals

Literals are notations for constant values of some built-in types. Some typical literals are string literalsCharacter string bytes literalsImmutable sequence of single byte bytearray literalsMutable sequence of bytes numeric literalsNumeric Value integer literalsInteger Value floating point literalsFloating point number imaginary literalsImaginary boolean literalsBoolean value list literalsMutable sequence of objects tuple literalsImmutable sequence of objects dictionary literalsMutable associative mapping (aka dictionary) set literalsMutable unordered set of distinct objects frozenset literalsImmutable form of set class

Operators

An operator is a symbol that performs an operation on one or more operands.

Operators::="+" | "-" | "*" | "**" | "/" | "//" | "%" | "@" | "<<" | ">>" | "&" | "|" | "^" | "~" | ":=" | "<" | ">" | "<=" | ">=" | "==" | "!="

Delimiters

A delimiter is a sequence of one or more characters used to specify the boundary between independent regions or elements.

Operators::="(" | ")" | "[" | "]" | "{" | "}" | "," | ":" | "." | ";" | "@" | "=" | "->" | "+=" | "-=" | "*=" | "/=" | "//=" | "%=" | "@=" | "&=" | "|=" | "^=" | "<<=" | ">>=" | "**="

The period can also occur in floating-point and imaginary literals. A sequence of three periods has a special meaning as an ellipsis literal. The second half of the list, the augmented assignment operators, serve lexically as delimiters, but also perform an operation. The printing ASCII characters, ', ", #, and \, have special meaning as part of other tokens or are otherwise significant to the lexical analyzer. The printing ASCII characters, $ and ?, are not used as operators in Python, but can be used in string literals and comments.

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