Python 3
Python Language Reference, §2.3, “Identifiers and keywords”
The syntax of identifiers in Python is based on the Unicode standard annex UAX-31, with elaboration and changes as defined below; see also PEP 3131 for further details.
Within the ASCII range (U+0001..U+007F), the valid characters for identifiers are the same as in Python 2.x: the uppercase and lowercase letters A through Z, the underscore _ and, except for the first character, the digits 0 through 9.
Python 3.0 introduces additional characters from outside the ASCII range (see PEP 3131). For these characters, the classification uses the version of the Unicode Character Database as included in the unicodedata module.
Identifiers are unlimited in length. Case is significant.
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 number
- Pc – connector punctuations
- Other_ID_Start – explicit list of characters in PropList.txt to support backwards compatibility
- Other_ID_Continue – likewise
All identifiers are converted into the normal form NFKC while parsing; comparison of identifiers is based on NFKC.
A non-normative HTML file listing all valid identifier characters for Unicode 4.1 can be found at https://www.dcl.hpi.uni-potsdam.de/home/loewis/table-3131.html.
Python 2
Python Language Reference, §2.3, “Identifiers and keywords”
Identifiers (also referred to as names) are described by the following lexical definitions:
identifier ::= (letter|"_") (letter | digit | "_")* letter ::= lowercase | uppercase lowercase ::= "a"..."z" uppercase ::= "A"..."Z" digit ::= "0"..."9"Identifiers are unlimited in length. Case is significant.