1 2.. _lexical: 3 4**************** 5Lexical analysis 6**************** 7 8.. index:: 9 single: lexical analysis 10 single: parser 11 single: token 12 13A Python program is read by a *parser*. Input to the parser is a stream of 14*tokens*, generated by the *lexical analyzer*. This chapter describes how the 15lexical analyzer breaks a file into tokens. 16 17Python uses the 7-bit ASCII character set for program text. 18 19.. versionadded:: 2.3 20 An encoding declaration can be used to indicate that string literals and 21 comments use an encoding different from ASCII. 22 23For compatibility with older versions, Python only warns if it finds 8-bit 24characters; those warnings should be corrected by either declaring an explicit 25encoding, or using escape sequences if those bytes are binary data, instead of 26characters. 27 28The run-time character set depends on the I/O devices connected to the program 29but is generally a superset of ASCII. 30 31**Future compatibility note:** It may be tempting to assume that the character 32set for 8-bit characters is ISO Latin-1 (an ASCII superset that covers most 33western languages that use the Latin alphabet), but it is possible that in the 34future Unicode text editors will become common. These generally use the UTF-8 35encoding, which is also an ASCII superset, but with very different use for the 36characters with ordinals 128-255. While there is no consensus on this subject 37yet, it is unwise to assume either Latin-1 or UTF-8, even though the current 38implementation appears to favor Latin-1. This applies both to the source 39character set and the run-time character set. 40 41 42.. _line-structure: 43 44Line structure 45============== 46 47.. index:: single: line structure 48 49A Python program is divided into a number of *logical lines*. 50 51 52.. _logical: 53 54Logical lines 55------------- 56 57.. index:: 58 single: logical line 59 single: physical line 60 single: line joining 61 single: NEWLINE token 62 63The end of a logical line is represented by the token NEWLINE. Statements 64cannot cross logical line boundaries except where NEWLINE is allowed by the 65syntax (e.g., between statements in compound statements). A logical line is 66constructed from one or more *physical lines* by following the explicit or 67implicit *line joining* rules. 68 69 70.. _physical: 71 72Physical lines 73-------------- 74 75A physical line is a sequence of characters terminated by an end-of-line 76sequence. In source files, any of the standard platform line termination 77sequences can be used - the Unix form using ASCII LF (linefeed), the Windows 78form using the ASCII sequence CR LF (return followed by linefeed), or the old 79Macintosh form using the ASCII CR (return) character. All of these forms can be 80used equally, regardless of platform. 81 82When embedding Python, source code strings should be passed to Python APIs using 83the standard C conventions for newline characters (the ``\n`` character, 84representing ASCII LF, is the line terminator). 85 86 87.. _comments: 88 89Comments 90-------- 91 92.. index:: 93 single: comment 94 single: hash character 95 96A comment starts with a hash character (``#``) that is not part of a string 97literal, and ends at the end of the physical line. A comment signifies the end 98of the logical line unless the implicit line joining rules are invoked. Comments 99are ignored by the syntax; they are not tokens. 100 101 102.. _encodings: 103 104Encoding declarations 105--------------------- 106 107.. index:: source character set, encoding declarations (source file) 108 109If a comment in the first or second line of the Python script matches the 110regular expression ``coding[=:]\s*([-\w.]+)``, this comment is processed as an 111encoding declaration; the first group of this expression names the encoding of 112the source code file. The encoding declaration must appear on a line of its 113own. If it is the second line, the first line must also be a comment-only line. 114The recommended forms of an encoding expression are :: 115 116 # -*- coding: <encoding-name> -*- 117 118which is recognized also by GNU Emacs, and :: 119 120 # vim:fileencoding=<encoding-name> 121 122which is recognized by Bram Moolenaar's VIM. In addition, if the first bytes of 123the file are the UTF-8 byte-order mark (``'\xef\xbb\xbf'``), the declared file 124encoding is UTF-8 (this is supported, among others, by Microsoft's 125:program:`notepad`). 126 127If an encoding is declared, the encoding name must be recognized by Python. The 128encoding is used for all lexical analysis, in particular to find the end of a 129string, and to interpret the contents of Unicode literals. String literals are 130converted to Unicode for syntactical analysis, then converted back to their 131original encoding before interpretation starts. 132 133.. XXX there should be a list of supported encodings. 134 135 136.. _explicit-joining: 137 138Explicit line joining 139--------------------- 140 141.. index:: 142 single: physical line 143 single: line joining 144 single: line continuation 145 single: backslash character 146 147Two or more physical lines may be joined into logical lines using backslash 148characters (``\``), as follows: when a physical line ends in a backslash that is 149not part of a string literal or comment, it is joined with the following forming 150a single logical line, deleting the backslash and the following end-of-line 151character. For example:: 152 153 if 1900 < year < 2100 and 1 <= month <= 12 \ 154 and 1 <= day <= 31 and 0 <= hour < 24 \ 155 and 0 <= minute < 60 and 0 <= second < 60: # Looks like a valid date 156 return 1 157 158A line ending in a backslash cannot carry a comment. A backslash does not 159continue a comment. A backslash does not continue a token except for string 160literals (i.e., tokens other than string literals cannot be split across 161physical lines using a backslash). A backslash is illegal elsewhere on a line 162outside a string literal. 163 164 165.. _implicit-joining: 166 167Implicit line joining 168--------------------- 169 170Expressions in parentheses, square brackets or curly braces can be split over 171more than one physical line without using backslashes. For example:: 172 173 month_names = ['Januari', 'Februari', 'Maart', # These are the 174 'April', 'Mei', 'Juni', # Dutch names 175 'Juli', 'Augustus', 'September', # for the months 176 'Oktober', 'November', 'December'] # of the year 177 178Implicitly continued lines can carry comments. The indentation of the 179continuation lines is not important. Blank continuation lines are allowed. 180There is no NEWLINE token between implicit continuation lines. Implicitly 181continued lines can also occur within triple-quoted strings (see below); in that 182case they cannot carry comments. 183 184 185.. _blank-lines: 186 187Blank lines 188----------- 189 190.. index:: single: blank line 191 192A logical line that contains only spaces, tabs, formfeeds and possibly a 193comment, is ignored (i.e., no NEWLINE token is generated). During interactive 194input of statements, handling of a blank line may differ depending on the 195implementation of the read-eval-print loop. In the standard implementation, an 196entirely blank logical line (i.e. one containing not even whitespace or a 197comment) terminates a multi-line statement. 198 199 200.. _indentation: 201 202Indentation 203----------- 204 205.. index:: 206 single: indentation 207 single: whitespace 208 single: leading whitespace 209 single: space 210 single: tab 211 single: grouping 212 single: statement grouping 213 214Leading whitespace (spaces and tabs) at the beginning of a logical line is used 215to compute the indentation level of the line, which in turn is used to determine 216the grouping of statements. 217 218First, tabs are replaced (from left to right) by one to eight spaces such that 219the total number of characters up to and including the replacement is a multiple 220of eight (this is intended to be the same rule as used by Unix). The total 221number of spaces preceding the first non-blank character then determines the 222line's indentation. Indentation cannot be split over multiple physical lines 223using backslashes; the whitespace up to the first backslash determines the 224indentation. 225 226**Cross-platform compatibility note:** because of the nature of text editors on 227non-UNIX platforms, it is unwise to use a mixture of spaces and tabs for the 228indentation in a single source file. It should also be noted that different 229platforms may explicitly limit the maximum indentation level. 230 231A formfeed character may be present at the start of the line; it will be ignored 232for the indentation calculations above. Formfeed characters occurring elsewhere 233in the leading whitespace have an undefined effect (for instance, they may reset 234the space count to zero). 235 236.. index:: 237 single: INDENT token 238 single: DEDENT token 239 240The indentation levels of consecutive lines are used to generate INDENT and 241DEDENT tokens, using a stack, as follows. 242 243Before the first line of the file is read, a single zero is pushed on the stack; 244this will never be popped off again. The numbers pushed on the stack will 245always be strictly increasing from bottom to top. At the beginning of each 246logical line, the line's indentation level is compared to the top of the stack. 247If it is equal, nothing happens. If it is larger, it is pushed on the stack, and 248one INDENT token is generated. If it is smaller, it *must* be one of the 249numbers occurring on the stack; all numbers on the stack that are larger are 250popped off, and for each number popped off a DEDENT token is generated. At the 251end of the file, a DEDENT token is generated for each number remaining on the 252stack that is larger than zero. 253 254Here is an example of a correctly (though confusingly) indented piece of Python 255code:: 256 257 def perm(l): 258 # Compute the list of all permutations of l 259 if len(l) <= 1: 260 return [l] 261 r = [] 262 for i in range(len(l)): 263 s = l[:i] + l[i+1:] 264 p = perm(s) 265 for x in p: 266 r.append(l[i:i+1] + x) 267 return r 268 269The following example shows various indentation errors:: 270 271 def perm(l): # error: first line indented 272 for i in range(len(l)): # error: not indented 273 s = l[:i] + l[i+1:] 274 p = perm(l[:i] + l[i+1:]) # error: unexpected indent 275 for x in p: 276 r.append(l[i:i+1] + x) 277 return r # error: inconsistent dedent 278 279(Actually, the first three errors are detected by the parser; only the last 280error is found by the lexical analyzer --- the indentation of ``return r`` does 281not match a level popped off the stack.) 282 283 284.. _whitespace: 285 286Whitespace between tokens 287------------------------- 288 289Except at the beginning of a logical line or in string literals, the whitespace 290characters space, tab and formfeed can be used interchangeably to separate 291tokens. Whitespace is needed between two tokens only if their concatenation 292could otherwise be interpreted as a different token (e.g., ab is one token, but 293a b is two tokens). 294 295 296.. _other-tokens: 297 298Other tokens 299============ 300 301Besides NEWLINE, INDENT and DEDENT, the following categories of tokens exist: 302*identifiers*, *keywords*, *literals*, *operators*, and *delimiters*. Whitespace 303characters (other than line terminators, discussed earlier) are not tokens, but 304serve to delimit tokens. Where ambiguity exists, a token comprises the longest 305possible string that forms a legal token, when read from left to right. 306 307 308.. _identifiers: 309 310Identifiers and keywords 311======================== 312 313.. index:: 314 single: identifier 315 single: name 316 317Identifiers (also referred to as *names*) are described by the following lexical 318definitions: 319 320.. productionlist:: 321 identifier: (`letter`|"_") (`letter` | `digit` | "_")* 322 letter: `lowercase` | `uppercase` 323 lowercase: "a"..."z" 324 uppercase: "A"..."Z" 325 digit: "0"..."9" 326 327Identifiers are unlimited in length. Case is significant. 328 329 330.. _keywords: 331 332Keywords 333-------- 334 335.. index:: 336 single: keyword 337 single: reserved word 338 339The following identifiers are used as reserved words, or *keywords* of the 340language, and cannot be used as ordinary identifiers. They must be spelled 341exactly as written here: 342 343.. sourcecode:: text 344 345 and del from not while 346 as elif global or with 347 assert else if pass yield 348 break except import print 349 class exec in raise 350 continue finally is return 351 def for lambda try 352 353.. versionchanged:: 2.4 354 :const:`None` became a constant and is now recognized by the compiler as a name 355 for the built-in object :const:`None`. Although it is not a keyword, you cannot 356 assign a different object to it. 357 358.. versionchanged:: 2.5 359 Using :keyword:`as` and :keyword:`with` as identifiers triggers a warning. To 360 use them as keywords, enable the ``with_statement`` future feature . 361 362.. versionchanged:: 2.6 363 :keyword:`as` and :keyword:`with` are full keywords. 364 365 366.. _id-classes: 367 368Reserved classes of identifiers 369------------------------------- 370 371Certain classes of identifiers (besides keywords) have special meanings. These 372classes are identified by the patterns of leading and trailing underscore 373characters: 374 375``_*`` 376 Not imported by ``from module import *``. The special identifier ``_`` is used 377 in the interactive interpreter to store the result of the last evaluation; it is 378 stored in the :mod:`__builtin__` module. When not in interactive mode, ``_`` 379 has no special meaning and is not defined. See section :ref:`import`. 380 381 .. note:: 382 383 The name ``_`` is often used in conjunction with internationalization; 384 refer to the documentation for the :mod:`gettext` module for more 385 information on this convention. 386 387``__*__`` 388 System-defined names. These names are defined by the interpreter and its 389 implementation (including the standard library). Current system names are 390 discussed in the :ref:`specialnames` section and elsewhere. More will likely 391 be defined in future versions of Python. *Any* use of ``__*__`` names, in 392 any context, that does not follow explicitly documented use, is subject to 393 breakage without warning. 394 395``__*`` 396 Class-private names. Names in this category, when used within the context of a 397 class definition, are re-written to use a mangled form to help avoid name 398 clashes between "private" attributes of base and derived classes. See section 399 :ref:`atom-identifiers`. 400 401 402.. _literals: 403 404Literals 405======== 406 407.. index:: 408 single: literal 409 single: constant 410 411Literals are notations for constant values of some built-in types. 412 413 414.. _strings: 415 416String literals 417--------------- 418 419.. index:: single: string literal 420 421String literals are described by the following lexical definitions: 422 423.. index:: single: ASCII@ASCII 424 425.. productionlist:: 426 stringliteral: [`stringprefix`](`shortstring` | `longstring`) 427 stringprefix: "r" | "u" | "ur" | "R" | "U" | "UR" | "Ur" | "uR" 428 : | "b" | "B" | "br" | "Br" | "bR" | "BR" 429 shortstring: "'" `shortstringitem`* "'" | '"' `shortstringitem`* '"' 430 longstring: "'''" `longstringitem`* "'''" 431 : | '"""' `longstringitem`* '"""' 432 shortstringitem: `shortstringchar` | `escapeseq` 433 longstringitem: `longstringchar` | `escapeseq` 434 shortstringchar: <any source character except "\" or newline or the quote> 435 longstringchar: <any source character except "\"> 436 escapeseq: "\" <any ASCII character> 437 438One syntactic restriction not indicated by these productions is that whitespace 439is not allowed between the :token:`stringprefix` and the rest of the string 440literal. The source character set is defined by the encoding declaration; it is 441ASCII if no encoding declaration is given in the source file; see section 442:ref:`encodings`. 443 444.. index:: 445 single: triple-quoted string 446 single: Unicode Consortium 447 single: string; Unicode 448 single: raw string 449 450In plain English: String literals can be enclosed in matching single quotes 451(``'``) or double quotes (``"``). They can also be enclosed in matching groups 452of three single or double quotes (these are generally referred to as 453*triple-quoted strings*). The backslash (``\``) character is used to escape 454characters that otherwise have a special meaning, such as newline, backslash 455itself, or the quote character. String literals may optionally be prefixed with 456a letter ``'r'`` or ``'R'``; such strings are called :dfn:`raw strings` and use 457different rules for interpreting backslash escape sequences. A prefix of 458``'u'`` or ``'U'`` makes the string a Unicode string. Unicode strings use the 459Unicode character set as defined by the Unicode Consortium and ISO 10646. Some 460additional escape sequences, described below, are available in Unicode strings. 461A prefix of ``'b'`` or ``'B'`` is ignored in Python 2; it indicates that the 462literal should become a bytes literal in Python 3 (e.g. when code is 463automatically converted with 2to3). A ``'u'`` or ``'b'`` prefix may be followed 464by an ``'r'`` prefix. 465 466In triple-quoted strings, unescaped newlines and quotes are allowed (and are 467retained), except that three unescaped quotes in a row terminate the string. (A 468"quote" is the character used to open the string, i.e. either ``'`` or ``"``.) 469 470.. index:: 471 single: physical line 472 single: escape sequence 473 single: Standard C 474 single: C 475 476Unless an ``'r'`` or ``'R'`` prefix is present, escape sequences in strings are 477interpreted according to rules similar to those used by Standard C. The 478recognized escape sequences are: 479 480+-----------------+---------------------------------+-------+ 481| Escape Sequence | Meaning | Notes | 482+=================+=================================+=======+ 483| ``\newline`` | Ignored | | 484+-----------------+---------------------------------+-------+ 485| ``\\`` | Backslash (``\``) | | 486+-----------------+---------------------------------+-------+ 487| ``\'`` | Single quote (``'``) | | 488+-----------------+---------------------------------+-------+ 489| ``\"`` | Double quote (``"``) | | 490+-----------------+---------------------------------+-------+ 491| ``\a`` | ASCII Bell (BEL) | | 492+-----------------+---------------------------------+-------+ 493| ``\b`` | ASCII Backspace (BS) | | 494+-----------------+---------------------------------+-------+ 495| ``\f`` | ASCII Formfeed (FF) | | 496+-----------------+---------------------------------+-------+ 497| ``\n`` | ASCII Linefeed (LF) | | 498+-----------------+---------------------------------+-------+ 499| ``\N{name}`` | Character named *name* in the | | 500| | Unicode database (Unicode only) | | 501+-----------------+---------------------------------+-------+ 502| ``\r`` | ASCII Carriage Return (CR) | | 503+-----------------+---------------------------------+-------+ 504| ``\t`` | ASCII Horizontal Tab (TAB) | | 505+-----------------+---------------------------------+-------+ 506| ``\uxxxx`` | Character with 16-bit hex value | \(1) | 507| | *xxxx* (Unicode only) | | 508+-----------------+---------------------------------+-------+ 509| ``\Uxxxxxxxx`` | Character with 32-bit hex value | \(2) | 510| | *xxxxxxxx* (Unicode only) | | 511+-----------------+---------------------------------+-------+ 512| ``\v`` | ASCII Vertical Tab (VT) | | 513+-----------------+---------------------------------+-------+ 514| ``\ooo`` | Character with octal value | (3,5) | 515| | *ooo* | | 516+-----------------+---------------------------------+-------+ 517| ``\xhh`` | Character with hex value *hh* | (4,5) | 518+-----------------+---------------------------------+-------+ 519 520.. index:: single: ASCII@ASCII 521 522Notes: 523 524(1) 525 Individual code units which form parts of a surrogate pair can be encoded using 526 this escape sequence. 527 528(2) 529 Any Unicode character can be encoded this way, but characters outside the Basic 530 Multilingual Plane (BMP) will be encoded using a surrogate pair if Python is 531 compiled to use 16-bit code units (the default). 532 533(3) 534 As in Standard C, up to three octal digits are accepted. 535 536(4) 537 Unlike in Standard C, exactly two hex digits are required. 538 539(5) 540 In a string literal, hexadecimal and octal escapes denote the byte with the 541 given value; it is not necessary that the byte encodes a character in the source 542 character set. In a Unicode literal, these escapes denote a Unicode character 543 with the given value. 544 545.. index:: single: unrecognized escape sequence 546 547Unlike Standard C, all unrecognized escape sequences are left in the string 548unchanged, i.e., *the backslash is left in the string*. (This behavior is 549useful when debugging: if an escape sequence is mistyped, the resulting output 550is more easily recognized as broken.) It is also important to note that the 551escape sequences marked as "(Unicode only)" in the table above fall into the 552category of unrecognized escapes for non-Unicode string literals. 553 554When an ``'r'`` or ``'R'`` prefix is present, a character following a backslash 555is included in the string without change, and *all backslashes are left in the 556string*. For example, the string literal ``r"\n"`` consists of two characters: 557a backslash and a lowercase ``'n'``. String quotes can be escaped with a 558backslash, but the backslash remains in the string; for example, ``r"\""`` is a 559valid string literal consisting of two characters: a backslash and a double 560quote; ``r"\"`` is not a valid string literal (even a raw string cannot end in 561an odd number of backslashes). Specifically, *a raw string cannot end in a 562single backslash* (since the backslash would escape the following quote 563character). Note also that a single backslash followed by a newline is 564interpreted as those two characters as part of the string, *not* as a line 565continuation. 566 567When an ``'r'`` or ``'R'`` prefix is used in conjunction with a ``'u'`` or 568``'U'`` prefix, then the ``\uXXXX`` and ``\UXXXXXXXX`` escape sequences are 569processed while *all other backslashes are left in the string*. For example, 570the string literal ``ur"\u0062\n"`` consists of three Unicode characters: 'LATIN 571SMALL LETTER B', 'REVERSE SOLIDUS', and 'LATIN SMALL LETTER N'. Backslashes can 572be escaped with a preceding backslash; however, both remain in the string. As a 573result, ``\uXXXX`` escape sequences are only recognized when there are an odd 574number of backslashes. 575 576 577.. _string-catenation: 578 579String literal concatenation 580---------------------------- 581 582Multiple adjacent string literals (delimited by whitespace), possibly using 583different quoting conventions, are allowed, and their meaning is the same as 584their concatenation. Thus, ``"hello" 'world'`` is equivalent to 585``"helloworld"``. This feature can be used to reduce the number of backslashes 586needed, to split long strings conveniently across long lines, or even to add 587comments to parts of strings, for example:: 588 589 re.compile("[A-Za-z_]" # letter or underscore 590 "[A-Za-z0-9_]*" # letter, digit or underscore 591 ) 592 593Note that this feature is defined at the syntactical level, but implemented at 594compile time. The '+' operator must be used to concatenate string expressions 595at run time. Also note that literal concatenation can use different quoting 596styles for each component (even mixing raw strings and triple quoted strings). 597 598 599.. _numbers: 600 601Numeric literals 602---------------- 603 604.. index:: 605 single: number 606 single: numeric literal 607 single: integer literal 608 single: plain integer literal 609 single: long integer literal 610 single: floating point literal 611 single: hexadecimal literal 612 single: binary literal 613 single: octal literal 614 single: decimal literal 615 single: imaginary literal 616 single: complex; literal 617 618There are four types of numeric literals: plain integers, long integers, 619floating point numbers, and imaginary numbers. There are no complex literals 620(complex numbers can be formed by adding a real number and an imaginary number). 621 622Note that numeric literals do not include a sign; a phrase like ``-1`` is 623actually an expression composed of the unary operator '``-``' and the literal 624``1``. 625 626 627.. _integers: 628 629Integer and long integer literals 630--------------------------------- 631 632Integer and long integer literals are described by the following lexical 633definitions: 634 635.. productionlist:: 636 longinteger: `integer` ("l" | "L") 637 integer: `decimalinteger` | `octinteger` | `hexinteger` | `bininteger` 638 decimalinteger: `nonzerodigit` `digit`* | "0" 639 octinteger: "0" ("o" | "O") `octdigit`+ | "0" `octdigit`+ 640 hexinteger: "0" ("x" | "X") `hexdigit`+ 641 bininteger: "0" ("b" | "B") `bindigit`+ 642 nonzerodigit: "1"..."9" 643 octdigit: "0"..."7" 644 bindigit: "0" | "1" 645 hexdigit: `digit` | "a"..."f" | "A"..."F" 646 647Although both lower case ``'l'`` and upper case ``'L'`` are allowed as suffix 648for long integers, it is strongly recommended to always use ``'L'``, since the 649letter ``'l'`` looks too much like the digit ``'1'``. 650 651Plain integer literals that are above the largest representable plain integer 652(e.g., 2147483647 when using 32-bit arithmetic) are accepted as if they were 653long integers instead. [#]_ There is no limit for long integer literals apart 654from what can be stored in available memory. 655 656Some examples of plain integer literals (first row) and long integer literals 657(second and third rows):: 658 659 7 2147483647 0177 660 3L 79228162514264337593543950336L 0377L 0x100000000L 661 79228162514264337593543950336 0xdeadbeef 662 663 664.. _floating: 665 666Floating point literals 667----------------------- 668 669Floating point literals are described by the following lexical definitions: 670 671.. productionlist:: 672 floatnumber: `pointfloat` | `exponentfloat` 673 pointfloat: [`intpart`] `fraction` | `intpart` "." 674 exponentfloat: (`intpart` | `pointfloat`) `exponent` 675 intpart: `digit`+ 676 fraction: "." `digit`+ 677 exponent: ("e" | "E") ["+" | "-"] `digit`+ 678 679Note that the integer and exponent parts of floating point numbers can look like 680octal integers, but are interpreted using radix 10. For example, ``077e010`` is 681legal, and denotes the same number as ``77e10``. The allowed range of floating 682point literals is implementation-dependent. Some examples of floating point 683literals:: 684 685 3.14 10. .001 1e100 3.14e-10 0e0 686 687Note that numeric literals do not include a sign; a phrase like ``-1`` is 688actually an expression composed of the unary operator ``-`` and the literal 689``1``. 690 691 692.. _imaginary: 693 694Imaginary literals 695------------------ 696 697Imaginary literals are described by the following lexical definitions: 698 699.. productionlist:: 700 imagnumber: (`floatnumber` | `intpart`) ("j" | "J") 701 702An imaginary literal yields a complex number with a real part of 0.0. Complex 703numbers are represented as a pair of floating point numbers and have the same 704restrictions on their range. To create a complex number with a nonzero real 705part, add a floating point number to it, e.g., ``(3+4j)``. Some examples of 706imaginary literals:: 707 708 3.14j 10.j 10j .001j 1e100j 3.14e-10j 709 710 711.. _operators: 712 713Operators 714========= 715 716.. index:: single: operators 717 718The following tokens are operators: 719 720.. code-block:: none 721 722 723 + - * ** / // % 724 << >> & | ^ ~ 725 < > <= >= == != <> 726 727The comparison operators ``<>`` and ``!=`` are alternate spellings of the same 728operator. ``!=`` is the preferred spelling; ``<>`` is obsolescent. 729 730 731.. _delimiters: 732 733Delimiters 734========== 735 736.. index:: single: delimiters 737 738The following tokens serve as delimiters in the grammar: 739 740.. code-block:: none 741 742 ( ) [ ] { } @ 743 , : . ` = ; 744 += -= *= /= //= %= 745 &= |= ^= >>= <<= **= 746 747The period can also occur in floating-point and imaginary literals. A sequence 748of three periods has a special meaning as an ellipsis in slices. The second half 749of the list, the augmented assignment operators, serve lexically as delimiters, 750but also perform an operation. 751 752The following printing ASCII characters have special meaning as part of other 753tokens or are otherwise significant to the lexical analyzer: 754 755.. code-block:: none 756 757 ' " # \ 758 759.. index:: single: ASCII@ASCII 760 761The following printing ASCII characters are not used in Python. Their 762occurrence outside string literals and comments is an unconditional error: 763 764.. code-block:: none 765 766 $ ? 767 768.. rubric:: Footnotes 769 770.. [#] In versions of Python prior to 2.4, octal and hexadecimal literals in the range 771 just above the largest representable plain integer but below the largest 772 unsigned 32-bit number (on a machine using 32-bit arithmetic), 4294967296, were 773 taken as the negative plain integer obtained by subtracting 4294967296 from 774 their unsigned value. 775 776