2.1.1.Logical lines¶

The end of a logical line is represented by the token NEWLINE. Statementscannot cross logical line boundaries except where NEWLINE is allowed by thesyntax (e.g., between statements in compound statements). A logical line isconstructed from one or morephysical lines by following the explicit orimplicitline joining rules.

2.1.2.Physical lines¶

A physical line is a sequence of characters terminated by an end-of-linesequence. In source files and strings, any of the standard platform linetermination sequences can be used - the Unix form using ASCII LF (linefeed),the Windows form using the ASCII sequence CR LF (return followed by linefeed),or the old Macintosh form using the ASCII CR (return) character. All of theseforms can be used equally, regardless of platform. The end of input also servesas an implicit terminator for the final physical line.

When embedding Python, source code strings should be passed to Python APIs usingthe standard C conventions for newline characters (the\n character,representing ASCII LF, is the line terminator).

2.1.3.Comments¶

A comment starts with a hash character (#) that is not part of a stringliteral, and ends at the end of the physical line. A comment signifies the endof the logical line unless the implicit line joining rules are invoked. Commentsare ignored by the syntax.

2.1.4.Encoding declarations¶

If a comment in the first or second line of the Python script matches theregular expressioncoding[=:]\s*([-\w.]+), this comment is processed as anencoding declaration; the first group of this expression names the encoding ofthe source code file. The encoding declaration must appear on a line of itsown. If it is the second line, the first line must also be a comment-only line.The recommended forms of an encoding expression are

# -*- coding: <encoding-name> -*-

which is recognized also by GNU Emacs, and

# vim:fileencoding=<encoding-name>

which is recognized by Bram Moolenaar’s VIM.

If no encoding declaration is found, the default encoding is UTF-8. If theimplicit or explicit encoding of a file is UTF-8, an initial UTF-8 byte-ordermark (b'\xef\xbb\xbf') is ignored rather than being a syntax error.

If an encoding is declared, the encoding name must be recognized by Python(seeStandard Encodings). Theencoding is used for all lexical analysis, including string literals, commentsand identifiers.

2.1.5.Explicit line joining¶

Two or more physical lines may be joined into logical lines using backslashcharacters (\), as follows: when a physical line ends in a backslash that isnot part of a string literal or comment, it is joined with the following forminga single logical line, deleting the backslash and the following end-of-linecharacter. For example:

if1900<year<2100and1<=month<=12 \and1<=day<=31and0<=hour<24 \and0<=minute<60and0<=second<60:# Looks like a valid datereturn1

A line ending in a backslash cannot carry a comment. A backslash does notcontinue a comment. A backslash does not continue a token except for stringliterals (i.e., tokens other than string literals cannot be split acrossphysical lines using a backslash). A backslash is illegal elsewhere on a lineoutside a string literal.

2.1.6.Implicit line joining¶

Expressions in parentheses, square brackets or curly braces can be split overmore than one physical line without using backslashes. For example:

month_names=['Januari','Februari','Maart',# These are the'April','Mei','Juni',# Dutch names'Juli','Augustus','September',# for the months'Oktober','November','December']# of the year

Implicitly continued lines can carry comments. The indentation of thecontinuation lines is not important. Blank continuation lines are allowed.There is no NEWLINE token between implicit continuation lines. Implicitlycontinued lines can also occur within triple-quoted strings (see below); in thatcase they cannot carry comments.

2.1.7.Blank lines¶

A logical line that contains only spaces, tabs, formfeeds and possibly acomment, is ignored (i.e., no NEWLINE token is generated). During interactiveinput of statements, handling of a blank line may differ depending on theimplementation of the read-eval-print loop. In the standard interactiveinterpreter, an entirely blank logical line (i.e. one containing not evenwhitespace or a comment) terminates a multi-line statement.

2.1.8.Indentation¶

Leading whitespace (spaces and tabs) at the beginning of a logical line is usedto compute the indentation level of the line, which in turn is used to determinethe grouping of statements.

Tabs are replaced (from left to right) by one to eight spaces such that thetotal number of characters up to and including the replacement is a multiple ofeight (this is intended to be the same rule as used by Unix). The total numberof spaces preceding the first non-blank character then determines the line’sindentation. Indentation cannot be split over multiple physical lines usingbackslashes; the whitespace up to the first backslash determines theindentation.

Indentation is rejected as inconsistent if a source file mixes tabs and spacesin a way that makes the meaning dependent on the worth of a tab in spaces; aTabError is raised in that case.

Cross-platform compatibility note: because of the nature of text editors onnon-UNIX platforms, it is unwise to use a mixture of spaces and tabs for theindentation in a single source file. It should also be noted that differentplatforms may explicitly limit the maximum indentation level.

A formfeed character may be present at the start of the line; it will be ignoredfor the indentation calculations above. Formfeed characters occurring elsewherein the leading whitespace have an undefined effect (for instance, they may resetthe space count to zero).

The indentation levels of consecutive lines are used to generate INDENT andDEDENT tokens, using a stack, as follows.

Before the first line of the file is read, a single zero is pushed on the stack;this will never be popped off again. The numbers pushed on the stack willalways be strictly increasing from bottom to top. At the beginning of eachlogical line, the line’s indentation level is compared to the top of the stack.If it is equal, nothing happens. If it is larger, it is pushed on the stack, andone INDENT token is generated. If it is smaller, itmust be one of thenumbers occurring on the stack; all numbers on the stack that are larger arepopped off, and for each number popped off a DEDENT token is generated. At theend of the file, a DEDENT token is generated for each number remaining on thestack that is larger than zero.

Here is an example of a correctly (though confusingly) indented piece of Pythoncode:

defperm(l):# Compute the list of all permutations of liflen(l)<=1:return[l]r=[]foriinrange(len(l)):s=l[:i]+l[i+1:]p=perm(s)forxinp:r.append(l[i:i+1]+x)returnr

The following example shows various indentation errors:

defperm(l):# error: first line indentedforiinrange(len(l)):# error: not indenteds=l[:i]+l[i+1:]p=perm(l[:i]+l[i+1:])# error: unexpected indentforxinp:r.append(l[i:i+1]+x)returnr# error: inconsistent dedent

(Actually, the first three errors are detected by the parser; only the lasterror is found by the lexical analyzer — the indentation ofreturnr doesnot match a level popped off the stack.)

2.1.9.Whitespace between tokens¶

Except at the beginning of a logical line or in string literals, the whitespacecharacters space, tab and formfeed can be used interchangeably to separatetokens. Whitespace is needed between two tokens only if their concatenationcould otherwise be interpreted as a different token (e.g., ab is one token, buta b is two tokens).

2.3.1.Keywords¶

The following identifiers are used as reserved words, orkeywords of thelanguage, and cannot be used as ordinary identifiers. They must be spelledexactly as written here:

False      await      else       import     passNone       break      except     in         raiseTrue       class      finally    is         returnand        continue   for        lambda     tryas         def        from       nonlocal   whileassert     del        global     not        withasync      elif       if         or         yield

2.3.2.Soft Keywords¶

Some identifiers are only reserved under specific contexts. These are known assoft keywords. The identifiersmatch,case,type and_ cansyntactically act as keywords in certain contexts,but this distinction is done at the parser level, not when tokenizing.

As soft keywords, their use in the grammar is possible while stillpreserving compatibility with existing code that uses these names asidentifier names.

match,case, and_ are used in thematch statement.type is used in thetype statement.

2.3.3.Reserved classes of identifiers¶

Certain classes of identifiers (besides keywords) have special meanings. Theseclasses are identified by the patterns of leading and trailing underscorecharacters:

_*

Not imported byfrommoduleimport*.

_

In acase pattern within amatch statement,_ is asoft keyword that denotes awildcard.

Separately, the interactive interpreter makes the result of the last evaluationavailable in the variable_.(It is stored in thebuiltins module, alongside built-infunctions likeprint.)

Elsewhere,_ is a regular identifier. It is often used to name“special” items, but it is not special to Python itself.

Note

The name_ is often used in conjunction with internationalization;refer to the documentation for thegettext module for moreinformation on this convention.

It is also commonly used for unused variables.

__*__

System-defined names, informally known as “dunder” names. These names aredefined by the interpreter and its implementation (including the standard library).Current system names are discussed in theSpecial method names section and elsewhere.More will likely be defined in future versions of Python.Any use of__*__ names,in any context, that does not follow explicitly documented use, is subject tobreakage without warning.

__*

Class-private names. Names in this category, when used within the context of aclass definition, are re-written to use a mangled form to help avoid nameclashes between “private” attributes of base and derived classes. See sectionIdentifiers (Names).

2.4.1.String and Bytes literals¶

String literals are described by the following lexical definitions:

stringliteral   ::= [stringprefix](shortstring |longstring)stringprefix    ::= "r" | "u" | "R" | "U" | "f" | "F"                    | "fr" | "Fr" | "fR" | "FR" | "rf" | "rF" | "Rf" | "RF"shortstring     ::= "'"shortstringitem* "'" | '"'shortstringitem* '"'longstring      ::= "'''"longstringitem* "'''" | '"""'longstringitem* '"""'shortstringitem ::=shortstringchar |stringescapeseqlongstringitem  ::=longstringchar |stringescapeseqshortstringchar ::= <any source character except "\" or newline or the quote>longstringchar  ::= <any source character except "\">stringescapeseq ::= "\" <any source character>

bytesliteral   ::=bytesprefix(shortbytes |longbytes)bytesprefix    ::= "b" | "B" | "br" | "Br" | "bR" | "BR" | "rb" | "rB" | "Rb" | "RB"shortbytes     ::= "'"shortbytesitem* "'" | '"'shortbytesitem* '"'longbytes      ::= "'''"longbytesitem* "'''" | '"""'longbytesitem* '"""'shortbytesitem ::=shortbyteschar |bytesescapeseqlongbytesitem  ::=longbyteschar |bytesescapeseqshortbyteschar ::= <any ASCII character except "\" or newline or the quote>longbyteschar  ::= <any ASCII character except "\">bytesescapeseq ::= "\" <any ASCII character>

One syntactic restriction not indicated by these productions is that whitespaceis not allowed between thestringprefix orbytesprefix and the rest of the literal. The sourcecharacter set is defined by the encoding declaration; it is UTF-8 if no encodingdeclaration is given in the source file; see sectionEncoding declarations.

In plain English: Both types of literals can be enclosed in matching single quotes(') or double quotes ("). They can also be enclosed in matching groupsof three single or double quotes (these are generally referred to astriple-quoted strings). The backslash (\) character is used to give specialmeaning to otherwise ordinary characters liken, which means ‘newline’ whenescaped (\n). It can also be used to escape characters that otherwise have aspecial meaning, such as newline, backslash itself, or the quote character.Seeescape sequences below for examples.

Bytes literals are always prefixed with'b' or'B'; they produce aninstance of thebytes type instead of thestr type. Theymay only contain ASCII characters; bytes with a numeric value of 128 or greatermust be expressed with escapes.

Both string and bytes literals may optionally be prefixed with a letter'r'or'R'; such constructs are calledraw string literalsandraw bytes literals respectively and treat backslashes asliteral characters. As a result, in raw string literals,'\U' and'\u'escapes are not treated specially.

A string literal with'f' or'F' in its prefix is aformatted string literal; seef-strings. The'f' may becombined with'r', but not with'b' or'u', therefore rawformatted strings are possible, but formatted bytes literals are not.

In triple-quoted literals, unescaped newlines and quotes are allowed (and areretained), except that three unescaped quotes in a row terminate the literal. (A“quote” is the character used to open the literal, i.e. either' or".)

2.4.2.String literal concatenation¶

Multiple adjacent string or bytes literals (delimited by whitespace), possiblyusing different quoting conventions, are allowed, and their meaning is the sameas their concatenation. Thus,"hello"'world' is equivalent to"helloworld". This feature can be used to reduce the number of backslashesneeded, to split long strings conveniently across long lines, or even to addcomments to parts of strings, for example:

re.compile("[A-Za-z_]"# letter or underscore"[A-Za-z0-9_]*"# letter, digit or underscore)

Note that this feature is defined at the syntactical level, but implemented atcompile time. The ‘+’ operator must be used to concatenate string expressionsat run time. Also note that literal concatenation can use different quotingstyles for each component (even mixing raw strings and triple quoted strings),and formatted string literals may be concatenated with plain string literals.

2.4.3.f-strings¶

Aformatted string literal orf-string is a string literalthat is prefixed with'f' or'F'. These strings may containreplacement fields, which are expressions delimited by curly braces{}.While other string literals always have a constant value, formatted stringsare really expressions evaluated at run time.

Escape sequences are decoded like in ordinary string literals (except whena literal is also marked as a raw string). After decoding, the grammarfor the contents of the string is:

f_string          ::= (literal_char | "{{" | "}}" |replacement_field)*replacement_field ::= "{"f_expression ["="] ["!"conversion] [":"format_spec] "}"f_expression      ::= (conditional_expression | "*"or_expr)                      (","conditional_expression | "," "*"or_expr)* [","]                      |yield_expressionconversion        ::= "s" | "r" | "a"format_spec       ::= (literal_char |replacement_field)*literal_char      ::= <any code point except "{", "}" or NULL>

The parts of the string outside curly braces are treated literally,except that any doubled curly braces'{{' or'}}' are replacedwith the corresponding single curly brace. A single opening curlybracket'{' marks a replacement field, which starts with aPython expression. To display both the expression text and its value afterevaluation, (useful in debugging), an equal sign'=' may be added after theexpression. A conversion field, introduced by an exclamation point'!' mayfollow. A format specifier may also be appended, introduced by a colon':'.A replacement field ends with a closing curly bracket'}'.

Expressions in formatted string literals are treated like regularPython expressions surrounded by parentheses, with a few exceptions.An empty expression is not allowed, and bothlambda andassignment expressions:= must be surrounded by explicit parentheses.Each expression is evaluated in the context where the formatted string literalappears, in order from left to right. Replacement expressions can containnewlines in both single-quoted and triple-quoted f-strings and they can containcomments. Everything that comes after a# inside a replacement fieldis a comment (even closing braces and quotes). In that case, replacement fieldsmust be closed in a different line.

>>> f"abc{a # This is a comment }"... + 3}"'abc5'

When the equal sign'=' is provided, the output will have the expressiontext, the'=' and the evaluated value. Spaces after the opening brace'{', within the expression and after the'=' are all retained in theoutput. By default, the'=' causes therepr() of the expression to beprovided, unless there is a format specified. When a format is specified itdefaults to thestr() of the expression unless a conversion'!r' isdeclared.

If a conversion is specified, the result of evaluating the expressionis converted before formatting. Conversion'!s' callsstr() onthe result,'!r' callsrepr(), and'!a' callsascii().

The result is then formatted using theformat() protocol. Theformat specifier is passed to the__format__() method of theexpression or conversion result. An empty string is passed when theformat specifier is omitted. The formatted result is then included inthe final value of the whole string.

Top-level format specifiers may include nested replacement fields. These nestedfields may include their own conversion fields andformat specifiers, but may not include more deeply nested replacement fields. Theformat specifier mini-language is the same as that used bythestr.format() method.

Formatted string literals may be concatenated, but replacement fieldscannot be split across literals.

Some examples of formatted string literals:

>>>name="Fred">>>f"He said his name is{name!r}.""He said his name is 'Fred'.">>>f"He said his name is{repr(name)}."# repr() is equivalent to !r"He said his name is 'Fred'.">>>width=10>>>precision=4>>>value=decimal.Decimal("12.34567")>>>f"result:{value:{width}.{precision}}"# nested fields'result:      12.35'>>>today=datetime(year=2017,month=1,day=27)>>>f"{today:%B %d, %Y}"# using date format specifier'January 27, 2017'>>>f"{today=:%B %d, %Y}"# using date format specifier and debugging'today=January 27, 2017'>>>number=1024>>>f"{number:#0x}"# using integer format specifier'0x400'>>>foo="bar">>>f"{foo= }"# preserves whitespace" foo = 'bar'">>>line="The mill's closed">>>f"{line= }"'line = "The mill\'s closed"'>>>f"{line= :20}""line = The mill's closed   ">>>f"{line= !r:20}"'line = "The mill\'s closed" '

Reusing the outer f-string quoting type inside a replacement field ispermitted:

>>>a=dict(x=2)>>>f"abc{a["x"]} def"'abc 2 def'

Backslashes are also allowed in replacement fields and are evaluated the sameway as in any other context:

>>>a=["a","b","c"]>>>print(f"List a contains:\n{"\n".join(a)}")List a contains:abc

Formatted string literals cannot be used as docstrings, even if they do notinclude expressions.

>>>deffoo():...f"Not a docstring"...>>>foo.__doc__isNoneTrue

See alsoPEP 498 for the proposal that added formatted string literals,andstr.format(), which uses a related format string mechanism.

2.4.4.Numeric literals¶

There are three types of numeric literals: integers, floating-point numbers, andimaginary numbers. There are no complex literals (complex numbers can be formedby adding a real number and an imaginary number).

Note that numeric literals do not include a sign; a phrase like-1 isactually an expression composed of the unary operator ‘-’ and the literal1.

2.4.5.Integer literals¶

Integer literals are described by the following lexical definitions:

integer      ::=decinteger |bininteger |octinteger |hexintegerdecinteger   ::=nonzerodigit (["_"]digit)* | "0"+ (["_"] "0")*bininteger   ::= "0" ("b" | "B") (["_"]bindigit)+octinteger   ::= "0" ("o" | "O") (["_"]octdigit)+hexinteger   ::= "0" ("x" | "X") (["_"]hexdigit)+nonzerodigit ::= "1"..."9"digit        ::= "0"..."9"bindigit     ::= "0" | "1"octdigit     ::= "0"..."7"hexdigit     ::=digit | "a"..."f" | "A"..."F"

There is no limit for the length of integer literals apart from what can bestored in available memory.

Underscores are ignored for determining the numeric value of the literal. Theycan be used to group digits for enhanced readability. One underscore can occurbetween digits, and after base specifiers like0x.

Note that leading zeros in a non-zero decimal number are not allowed. This isfor disambiguation with C-style octal literals, which Python used before version3.0.

Some examples of integer literals:

721474836470o1770b1001101113792281625142643375935439503360o3770xdeadbeef100_000_000_0000b_1110_0101

2.4.6.Floating-point literals¶

Floating-point literals are described by the following lexical definitions:

floatnumber   ::=pointfloat |exponentfloatpointfloat    ::= [digitpart]fraction |digitpart "."exponentfloat ::= (digitpart |pointfloat)exponentdigitpart     ::=digit (["_"]digit)*fraction      ::= "."digitpartexponent      ::= ("e" | "E") ["+" | "-"]digitpart

Note that the integer and exponent parts are always interpreted using radix 10.For example,077e010 is legal, and denotes the same number as77e10. Theallowed range of floating-point literals is implementation-dependent. As ininteger literals, underscores are supported for digit grouping.

Some examples of floating-point literals:

3.1410..0011e1003.14e-100e03.14_15_93

2.4.7.Imaginary literals¶

Imaginary literals are described by the following lexical definitions:

imagnumber ::= (floatnumber |digitpart) ("j" | "J")

An imaginary literal yields a complex number with a real part of 0.0. Complexnumbers are represented as a pair of floating-point numbers and have the samerestrictions on their range. To create a complex number with a nonzero realpart, add a floating-point number to it, e.g.,(3+4j). Some examples ofimaginary literals:

3.14j10.j10j.001j1e100j3.14e-10j3.14_15_93j