Editing Python (programming language) (section)

===Expressions===
Python's [[expression (computer science)|expressions]] include the following:
* The <code>+</code>, <code>-</code>, and <code>*</code> operators for mathematical addition, subtraction, and multiplication are similar to other languages, but the behavior of division differs. There are two types of division in Python: [[floor division]] (or integer division) <code>//</code>, and floating-point division <code>/</code>.<ref>{{cite web|title=division|url=https://docs.python.org|website=python.org|access-date=30 July 2014|archive-date=20 July 2006|archive-url=https://web.archive.org/web/20060720033244/http://docs.python.org/|url-status=live}}</ref> Python uses the <code>**</code> operator for exponentiation.
* Python uses the <code>+</code> operator for string concatenation. The language uses the <code>*</code> operator for duplicating a string a specified number of times.
* The <code>@</code> infix operator<!-- was introduced in Python 3.5--> is intended to be used by libraries such as [[NumPy]] for [[matrix multiplication]].<ref name=PEP465>{{cite web |title=PEP 0465 – A dedicated infix operator for matrix multiplication |url=https://www.python.org/dev/peps/pep-0465/ |website=python.org |access-date=1 January 2016 |archive-date=4 June 2020 |archive-url=https://web.archive.org/web/20200604224255/https://www.python.org/dev/peps/pep-0465/ |url-status=live}}</ref><ref name=Python3.5Changelog>{{cite web |title=Python 3.5.1 Release and Changelog |url=https://www.python.org/downloads/release/python-351/ |website=python.org |access-date=1 January 2016 |archive-date=14 May 2020 |archive-url=https://web.archive.org/web/20200514034938/https://www.python.org/downloads/release/python-351/ |url-status=live}}</ref>
* The syntax <code>:=</code>, called the "walrus operator", was introduced in Python 3.8. This operator assigns values to variables as part of a larger expression.<ref name=Python3.8Changelog>{{cite web |title=What's New in Python 3.8 |url=https://docs.python.org/3.8/whatsnew/3.8.html |access-date=14 October 2019 |archive-date=8 June 2020 |archive-url=https://web.archive.org/web/20200608124345/https://docs.python.org/3.8/whatsnew/3.8.html |url-status=live}}</ref>
* In Python, <code>==</code> compares two objects by value. Python's <code>is</code> operator may be used to compare object identities (i.e., comparison by reference), and comparisons may be chained—for example, {{code|lang=python|code=a <= b <= c}}.
* Python uses <code>and</code>, <code>or</code>, and <code>not</code> as Boolean operators.
* Python has a type of expression called a ''[[List comprehension#Python|list comprehension]]'', and a more general expression called a ''generator expression''.<ref name="AutoNT-59"/>
* [[Anonymous function]]s are implemented using [[Lambda (programming)|lambda expressions]]; however, there may be only one expression in each body.
* Conditional expressions are written as {{code|lang=python|code=x if c else y}}.<ref name="AutoNT-60"/> (This is different in operand order from the <code>[[?:|c ? x : y]]</code> operator common to many other languages.)
* Python makes a distinction between [[list (computer science)|lists]] and [[tuple]]s. Lists are written as {{code|lang=python|code=[1, 2, 3]}}, are mutable, and cannot be used as the keys of dictionaries (since dictionary keys must be [[immutable]] in Python). Tuples, written as {{code|lang=python|code=(1, 2, 3)}}, are immutable and thus can be used as the keys of dictionaries, provided that all of the tuple's elements are immutable. The <code>+</code> operator can be used to concatenate two tuples, which does not directly modify their contents, but produces a new tuple containing the elements of both. For example, given the variable <code>t</code> initially equal to {{code|lang=python|code=(1, 2, 3)}}, executing {{code|lang=python|code=t = t + (4, 5)}} first evaluates {{code|lang=python|code=t + (4, 5)}}, which yields {{code|lang=python|code=(1, 2, 3, 4, 5)}}; this result is then assigned back to <code>t</code>—thereby effectively "modifying the contents" of <code>t</code> while conforming to the immutable nature of tuple objects. Parentheses are optional for tuples in unambiguous contexts.<ref>{{cite web|title=4. Built-in Types – Python 3.6.3rc1 documentation|url=https://docs.python.org/3/library/stdtypes.html#tuple|website=python.org|access-date=1 October 2017|archive-date=14 June 2020|archive-url=https://web.archive.org/web/20200614194325/https://docs.python.org/3/library/stdtypes.html#tuple|url-status=live}}</ref>
* Python features ''sequence unpacking'' where multiple expressions, each evaluating to something assignable (e.g., a variable or a writable property) are associated just as in forming tuple literal; as a whole, the results are then put on the left-hand side of the equal sign in an assignment statement. This statement expects an ''iterable'' object on the right-hand side of the equal sign to produce the same number of values as the writable expressions on the left-hand side; while iterating, the statement assigns each of the values produced on the right to the corresponding expression on the left.<ref>{{cite web|title=5.3. Tuples and Sequences – Python 3.7.1rc2 documentation|url=https://docs.python.org/3/tutorial/datastructures.html#tuples-and-sequences|website=python.org|access-date=17 October 2018|archive-date=10 June 2020|archive-url=https://web.archive.org/web/20200610050047/https://docs.python.org/3/tutorial/datastructures.html#tuples-and-sequences|url-status=live}}</ref>
* Python has a "string format" operator <code>%</code> that functions analogously to <code>[[printf]]</code> format strings in the C language—e.g. {{code|2=python|1="spam=%s eggs=%d" % ("blah", 2)}} evaluates to <code>"spam=blah eggs=2"</code>. In Python&nbsp;2.6+ and 3+, this operator was supplemented by the <code>format()</code> method of the <code>str</code> class, e.g., {{code|2=python|1="spam={0} eggs={1}".format("blah", 2)}}. Python&nbsp;3.6 added "f-strings": {{code|2=python|1=spam = "blah"; eggs = 2; f'spam={spam} eggs={eggs}'}}.<ref name="pep-0498">{{cite web |title=PEP 498 – Literal String Interpolation |url=https://www.python.org/dev/peps/pep-0498/ |website=python.org |access-date=8 March 2017 |archive-date=15 June 2020 |archive-url=https://web.archive.org/web/20200615184141/https://www.python.org/dev/peps/pep-0498/ |url-status=live}}</ref>
* Strings in Python can be [[concatenated]] by "adding" them (using the same operator as for adding integers and floats); e.g., {{code|2=python|1="spam" + "eggs"}} returns <code>"spameggs"</code>. If strings contain numbers, they are concatenated as strings rather than as integers, e.g. {{code|2=python|1="2" + "2"}} returns <code>"22"</code>.
* Python supports [[string literal]]s in several ways:
** Delimited by single or double quotation marks; single and double quotation marks have equivalent functionality (unlike in [[Unix shell]]s, [[Perl]], and Perl-influenced languages). Both marks use the backslash (<code>\</code>) as an [[escape character]]. [[String interpolation]] became available in Python&nbsp;3.6 as "formatted string literals".<ref name="pep-0498"/>
** Triple-quoted, i.e., starting and ending with three single or double quotation marks; this may span multiple lines and function like [[here document]]s in shells, Perl, and [[Ruby (programming language)|Ruby]].
** [[Raw string]] varieties, denoted by prefixing the string literal with <code>r</code>. Escape sequences are not interpreted; hence raw strings are useful where literal backslashes are common, such as in [[regular expression]]s and [[Windows]]-style paths. (Compare "<code>@</code>-quoting" in [[C Sharp (programming language)|C#]].)
* Python has [[array index]] and [[array slicing]] expressions in lists, which are written as <code>a[key]</code>, {{code|lang=python|code=a[start:stop]}} or {{code|lang=python|code=a[start:stop:step]}}. Indexes are [[zero-based numbering|zero-based]], and negative indexes are relative to the end. Slices take elements from the ''start'' index up to, but not including, the ''stop'' index. The (optional) third slice [[Parameter (computer programming)|parameter]], called ''step'' or ''stride'', allows elements to be skipped or reversed. Slice indexes may be omitted—for example, {{code|lang=python|code=a[:]}} returns a copy of the entire list. Each element of a slice is a [[shallow copy]].

In Python, a distinction between expressions and statements is rigidly enforced, in contrast to languages such as [[Common Lisp]], [[Scheme (programming language)|Scheme]], or [[Ruby (programming language)|Ruby]]. This distinction leads to duplicating some functionality, for example:
* [[List comprehensions]] vs. <code>for</code>-loops
* [[Conditional (programming)|Conditional]] expressions vs. <code>if</code> blocks
* The <code>eval()</code> vs. <code>exec()</code> built-in functions (in Python&nbsp;2, <code>exec</code> is a statement); the former function is for expressions, while the latter is for statements

A statement cannot be part of an expression; because of this restriction, expressions such as list and <code>dict</code> comprehensions (and lambda expressions) cannot contain statements. As a particular case, an assignment statement such as {{code|lang=python|code=a = 1}} cannot be part of the conditional expression of a conditional statement.