2.1.1.Logical lines¶

The end of a logical line is represented by the tokenNEWLINE.Statements cannot cross logical line boundaries except whereNEWLINEis allowed by the syntax (e.g., between statements in compound statements).A logical line is constructed from one or morephysical lines by followingtheexplicit orimplicitline joining rules.

2.1.2.Physical lines¶

A physical line is a sequence of characters terminated by one the followingend-of-line sequences:

• the Unix form using ASCII LF (linefeed),

• the Windows form using the ASCII sequence CR LF (return followed by linefeed),

• the ‘Classic Mac OS’ form using the ASCII CR (return) character.

Regardless of platform, each of these sequences is replaced by a singleASCII LF (linefeed) character.(This is done even insidestring literals.)Each line can use any of the sequences; they do not need to be consistentwithin a file.

The end of input also serves as an implicit terminator for the finalphysical line.

Formally:

newline: <ASCII LF> | <ASCII CR> <ASCII LF> | <ASCII CR>

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.

All lexical analysis, including string literals, commentsand identifiers, works on Unicode text decoded using the source encoding.Any Unicode code point, except the NUL control character, can appear inPython source.

source_character:  <any Unicode code point, except NUL>

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., noNEWLINE token is generated).During interactive input of statements, handling of a blank line may differdepending on the implementation of the read-eval-print loop.In the standard interactive interpreter, an entirely blank logical line (thatis, one containing not even whitespace or a comment) terminates a multi-linestatement.

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 generateINDENT 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, andoneINDENT 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 aDEDENT token is generated.At the end of the file, aDEDENT token is generated for each numberremaining on the stack 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. For example,ab is onetoken, butab is two tokens. However,+a and+a both producetwo tokens,+ anda, as+a is not a valid token.

2.1.10.End marker¶

At the end of non-interactive input, the lexical analyzer generates anENDMARKER token.

2.3.1.Keywords¶

The following names 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 names are only reserved under specific contexts. These are known assoft keywords:

• match,case, and_, when used in thematch statement.

• type, when used in thetype statement.

These syntactically act as keywords in their specific 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.

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).

2.5.1.Triple-quoted strings¶

Strings can also be enclosed in matching groups of three single or doublequotes.These are generally referred to astriple-quoted strings:

"""This is a triple-quoted string."""

In triple-quoted literals, unescaped quotes are allowed (and areretained), except that three unescaped quotes in a row terminate the literal,if they are of the same kind (' or") used at the start:

"""This string has "quotes" inside."""

Unescaped newlines are also allowed and retained:

'''This triple-quoted stringcontinues on the next line.'''

2.5.2.String prefixes¶

String literals can have an optionalprefix that influences how thecontent of the literal is parsed, for example:

b"data"f'{result=}'

The allowed prefixes are:

• b:Bytes literal

• r:Raw string

• f:Formatted string literal (“f-string”)

• t:Template string literal (“t-string”)

• u: No effect (allowed for backwards compatibility)

See the linked sections for details on each type.

Prefixes are case-insensitive (for example, ‘B’ works the same as ‘b’).The ‘r’ prefix can be combined with ‘f’, ‘t’ or ‘b’, so ‘fr’,‘rf’, ‘tr’, ‘rt’, ‘br’, and ‘rb’ are also valid prefixes.

2.5.3.Formal grammar¶

String literals, except“f-strings” and“t-strings”, are described by thefollowing lexical definitions.

These definitions usenegative lookaheads (!)to indicate that an ending quote ends the literal.

STRING:          [stringprefix] (stringcontent)stringprefix:    <("r" |"u" |"b" |"br" |"rb"), case-insensitive>stringcontent:   |"'''" ( !"'''"longstringitem)*"'''"   |'"""' ( !'"""'longstringitem)*'"""'   |"'" ( !"'"stringitem)*"'"   |'"' ( !'"'stringitem)*'"'stringitem:stringchar |stringescapeseqstringchar:      <anysource_character, except backslash and newline>longstringitem:stringitem | newlinestringescapeseq:"\" <anysource_character>

Note that as in all lexical definitions, whitespace is significant.In particular, the prefix (if any) must be immediately followed by the startingquote.

2.5.4.Escape sequences¶

Unless an ‘r’ or ‘R’ prefix is present, escape sequences in string andbytes literals are interpreted according to rules similar to those used byStandard C. The recognized escape sequences are:

>>>'This string will not include\...backslashes or newline characters.''This string will not include backslashes or newline characters.'

>>>print('C:\\Program Files')C:\Program Files

>>>print('\' and\"')' and "

>>>'\120''P'

>>>'\x50''P'

>>>'\N{LATIN CAPITAL LETTER P}''P'>>>'\N{SNAKE}''🐍'

>>>'\u1234''ሴ'>>>'\U0001f40d''🐍'

>>>print('\q')\q>>>list('\q')['\\', 'q']

2.5.5.Bytes literals¶

Bytes literals are always prefixed with ‘b’ or ‘B’; they produce aninstance of thebytes type instead of thestr type.They may only contain ASCII characters; bytes with a numeric value of 128or greater must be expressed with escape sequences (typicallyHexadecimal character orOctal character):

>>>b'\x89PNG\r\n\x1a\n'b'\x89PNG\r\n\x1a\n'>>>list(b'\x89PNG\r\n\x1a\n')[137, 80, 78, 71, 13, 10, 26, 10]

Similarly, a zero byte must be expressed using an escape sequence (typically\0 or\x00).

2.5.6.Raw string literals¶

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,escape sequencesare not treated specially:

>>>r'\d{4}-\d{2}-\d{2}''\\d{4}-\\d{2}-\\d{2}'

Even in a raw literal, quotes can be escaped with a backslash, but thebackslash remains in the result; for example,r"\"" is a valid stringliteral consisting of two characters: a backslash and a double quote;r"\"is not a valid string literal (even a raw string cannot end in an odd number ofbackslashes). Specifically,a raw literal cannot end in a single backslash(since the backslash would escape the following quote character). Note alsothat a single backslash followed by a newline is interpreted as those twocharacters as part of the literal,not as a line continuation.

2.5.7.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.

2.5.8.t-strings¶

Atemplate string literal ort-string is a string literalthat is prefixed with ‘t’ or ‘T’.These strings follow the same syntax and evaluation rules asformatted string literals, with the following differences:

• Rather than evaluating to astr object, template string literals evaluateto astring.templatelib.Template object.

• Theformat() protocol is not used.Instead, the format specifier and conversions (if any) are passed toa newInterpolation object that is createdfor each evaluated expression.It is up to code that processes the resultingTemplateobject to decide how to handle format specifiers and conversions.

• Format specifiers containing nested replacement fields are evaluated eagerly,prior to being passed to theInterpolation object.For instance, an interpolation of the form{amount:.{precision}f} willevaluate the inner expression{precision} to determine the value of theformat_spec attribute.Ifprecision were to be2, the resulting format specifierwould be'.2f'.

• When the equals sign'=' is provided in an interpolation expression,the text of the expression is appended to the literal string that precedesthe relevant interpolation.This includes the equals sign and any surrounding whitespace.TheInterpolation instance for the expression will be created asnormal, except thatconversion willbe set to ‘r’ (repr()) by default.If an explicit conversion or format specifier are provided,this will override the default behaviour.

2.6.1.Integer literals¶

Integer literals denote whole numbers. For example:

732147483647

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

7922816251426433759354395033679228162514264337593543950336

Underscores can be used to group digits for enhanced readability,and are ignored for determining the numeric value of the literal.For example, the following literals are equivalent:

100_000_000_0001000000000001_00_00_00_00_000

Underscores can only occur between digits.For example,_123,321_, and123__321 arenot valid literals.

Integers can be specified in binary (base 2), octal (base 8), or hexadecimal(base 16) using the prefixes0b,0o and0x, respectively.Hexadecimal digits 10 through 15 are represented by lettersA-F,case-insensitive. For example:

0b1001101110b_1110_01010o1770o3770xdeadbeef0xDead_Beef

An underscore can follow the base specifier.For example,0x_1f is a valid literal, but0_x1f and0x__1f arenot.

Leading zeros in a non-zero decimal number are not allowed.For example,0123 is not a valid literal.This is for disambiguation with C-style octal literals, which Python usedbefore version 3.0.

Formally, integer literals are described by the following lexical definitions:

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

2.6.2.Floating-point literals¶

Floating-point (float) literals, such as3.14 or1.5, denoteapproximations of real numbers.

They consist ofinteger andfraction parts, each composed of decimal digits.The parts are separated by a decimal point,.:

2.718284.0

Unlike in integer literals, leading zeros are allowed in the numeric parts.For example,077.010 is legal, and denotes the same number as77.10.

As in integer literals, single underscores may occur between digits to helpreadability:

96_485.332_1233.14_15_93

Either of these parts, but not both, can be empty. For example:

10.# (equivalent to 10.0).001# (equivalent to 0.001)

Optionally, the integer and fraction may be followed by anexponent:the lettere orE, followed by an optional sign,+ or-,and a number in the same format as the integer and fraction parts.Thee orE represents “times ten raised to the power of”:

1.0e3# (represents 1.0×10³, or 1000.0)1.166e-5# (represents 1.166×10⁻⁵, or 0.00001166)6.02214076e+23# (represents 6.02214076×10²³, or 602214076000000000000000.)

In floats with only integer and exponent parts, the decimal point may beomitted:

1e3# (equivalent to 1.e3 and 1.0e3)0e0# (equivalent to 0.)

Formally, floating-point literals are described by the followinglexical definitions:

floatnumber:   |digitpart"." [digitpart] [exponent]   |"."digitpart [exponent]   |digitpartexponentdigitpart:digit (["_"]digit)*exponent:  ("e" |"E") ["+" |"-"]digitpart

2.6.3.Imaginary literals¶

Python hascomplex number objects, but no complexliterals.Instead,imaginary literals denote complex numbers with a zeroreal part.

For example, in math, the complex number 3+4.2i is writtenas the real number 3 added to the imaginary number 4.2i.Python uses a similar syntax, except the imaginary unit is written asjrather thani:

3+4.2j

This is an expression composedof theinteger literal3,theoperator ‘+’,and theimaginary literal4.2j.Since these are three separate tokens, whitespace is allowed between them:

3+4.2j

No whitespace is allowedwithin each token.In particular, thej suffix, may not be separated from the numberbefore it.

The number before thej has the same syntax as a floating-point literal.Thus, the following are valid imaginary literals:

4.2j3.14j10.j.001j1e100j3.14e-10j3.14_15_93j

Unlike in a floating-point literal the decimal point can be omitted if theimaginary number only has an integer part.The number is still evaluated as a floating-point number, not an integer:

10j0j1000000000000000000000000j# equivalent to 1e+24j

Thej suffix is case-insensitive.That means you can useJ instead:

3.14J# equivalent to 3.14j

Formally, imaginary literals are described by the following lexical definition:

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