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In contrast, a character entity reference refers to a character by the name of an entity which has the desired character as its replacement text. The entity must either be predefined (built into the markup language) or explicitly declared in a Document Type Definition (DTD). The format is the same as for any entity reference: &name;
For example, one could define a dictionary having a string "toast" mapped to the integer 42 or vice versa. The keys in a dictionary must be of an immutable Python type, such as an integer or a string, because under the hood they are implemented via a hash function. This makes for much faster lookup times, but requires keys not change.
Python's is operator may be used to compare object identities (comparison by reference), and comparisons may be chained—for example, a <= b <= c. Python uses and, or, and not as Boolean operators. Python has a type of expression named a list comprehension, and a more general expression named a generator expression. [77]
However, not all abstract characters are encoded as a single Unicode character, and some abstract characters may be represented in Unicode by a sequence of two or more characters. For example, a Latin small letter "i" with an ogonek , a dot above , and an acute accent , which is required in Lithuanian , is represented by the character sequence ...
Terminal symbols are the concrete characters or strings of characters (for example keywords such as define, if, let, or void) from which syntactically valid programs are constructed. Syntax can be divided into context-free syntax and context-sensitive syntax. [7] Context-free syntax are rules directed by the metalanguage of the programming ...
A "character" may use any number of Unicode code points. [21] For instance an emoji flag character takes 8 bytes, since it is "constructed from a pair of Unicode scalar values" [22] (and those values are outside the BMP and require 4 bytes each). UTF-16 in no way assists in "counting characters" or in "measuring the width of a string".
[8] [9] For example, the Latin alphabet and Greek alphabet are both character sets. A coded character set is a character set mapped to a set of unique numbers. [9] For historical reasons, this is also often referred to as a code page. [8] A character repertoire is the set of characters that can be represented by a particular coded character set.
The basic definition of a dictionary does not mandate an order. To guarantee a fixed order of enumeration, ordered versions of the associative array are often used. There are two senses of an ordered dictionary: The order of enumeration is always deterministic for a given set of keys by sorting.