2.1 identifier and reserved word
Identifier is used to identify variable, function, class name, and the object module. Python identifier may contain letters (AZ, az), numbers (0-9), and the underscore character (_), but it has limitations the following aspects:
⑴ identifier of the first alphabet character must be a letter or underscore '_', and the name can not have spaces between variables.
⑵ Python identifier is case-sensitive, so the data Data identifiers are different.
⑶ In Python 3, non-ASCII identifier is permitted to use
⑷ reserved words can not as identifiers.
import keyword
print（keyword.kwlist）