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, any of the standard platform line terminationsequences can be used - the Unix form using ASCII LF (linefeed), the Windowsform using the ASCII sequence CR LF (return followed by linefeed), or the oldMacintosh form using the ASCII CR (return) character. All of these forms can beused equally, regardless of platform.

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; they are not tokens.

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 recommended forms of this 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. Inaddition, if the first bytes of the file are the UTF-8 byte-order mark(b'\xef\xbb\xbf'), the declared file encoding is UTF-8 (this is supported,among others, by Microsoft’snotepad).

If an encoding is declared, the encoding name must be recognized by Python. Theencoding is used for all lexical analysis, including string literals, commentsand identifiers. The encoding declaration must appear on a line of its own.

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      class      finally    is         returnNone       continue   for        lambda     tryTrue       def        from       nonlocal   whileand        del        global     not        withas         elif       if         or         yieldassert     else       import     passbreak      except     in         raise

2.3.2. 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*. The special identifier_ is usedin the interactive interpreter to store the result of the last evaluation; it isstored in thebuiltins module. When not in interactive mode,_has no special meaning and is not defined. See sectionThe import statement.

Note

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

__*__

System-defined names. These names are defined by the interpreter and itsimplementation (including the standard library). Current system names arediscussed in theSpecial method names section and elsewhere. More will likelybe defined in future versions of Python.Any use of__*__ names, inany 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"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 therest of the literal. The source character set is defined by the encodingdeclaration; it is UTF-8 if no encoding declaration 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 escapecharacters that otherwise have a special meaning, such as newline, backslashitself, or the quote character.

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.

As of Python 3.3 it is possible again to prefix unicode strings with au prefix to simplify maintenance of dual 2.x and 3.x codebases.

Both string and bytes literals may optionally be prefixed with a letter'r'or'R'; such strings are calledraw strings and treat backslashes asliteral characters. As a result, in string literals,'\U' and'\u'escapes in raw strings are not treated specially. Given that Python 2.x’s rawunicode literals behave differently than Python 3.x’s the'ur' syntaxis not supported.

New in version 3.3:The'rb' prefix of raw bytes literals has been added as a synonymof'br'.

New in version 3.3:Support for the unicode legacy literal (u'value') was reintroducedto simplify the maintenance of dual Python 2.x and 3.x codebases.SeePEP 414 for more information.

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

Unless an'r' or'R' prefix is present, escape sequences in strings areinterpreted according to rules similar to those used by Standard C. Therecognized escape sequences are:

Escape sequences only recognized in string literals are:

Notes:

1. As in Standard C, up to three octal digits are accepted.

2. Unlike in Standard C, exactly two hex digits are required.

3. In a bytes literal, hexadecimal and octal escapes denote the byte with thegiven value. In a string literal, these escapes denote a Unicode characterwith the given value.

4. Changed in version 3.3:Support for name aliases[1] has been added.

5. Individual code units which form parts of a surrogate pair can be encoded usingthis escape sequence. Exactly four hex digits are required.

6. Any Unicode character can be encoded this way. Exactly eight hex digitsare required.

Unlike Standard C, all unrecognized escape sequences are left in the stringunchanged, i.e.,the backslash is left in the string. (This behavior isuseful when debugging: if an escape sequence is mistyped, the resulting outputis more easily recognized as broken.) It is also important to note that theescape sequences only recognized in string literals fall into the category ofunrecognized escapes for bytes literals.

Even in a raw string, string quotes can be escaped with a backslash, but thebackslash remains in the string; 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 string 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 string,not as a line continuation.

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).

2.4.3. 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.

2.4.4. Integer literals¶

Integer literals are described by the following lexical definitions:

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

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

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:

721474836470o1770b1001101113792281625142643375935439503360o3770x100000000792281625142643375935439503360xdeadbeef

2.4.5. Floating point literals¶

Floating point literals are described by the following lexical definitions:

floatnumber ::=pointfloat |exponentfloatpointfloat ::=  [intpart]fraction |intpart "."exponentfloat ::=  (intpart |pointfloat)exponentintpart ::=digit+fraction ::=  "."digit+exponent ::=  ("e" | "E") ["+" | "-"]digit+

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. Someexamples of floating point literals:

3.1410..0011e1003.14e-100e0

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

2.4.6. Imaginary literals¶

Imaginary literals are described by the following lexical definitions:

imagnumber ::=  (floatnumber |intpart) ("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-10j

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