703 lines
26 KiB
ReStructuredText
703 lines
26 KiB
ReStructuredText
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.. _lexical:
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****************
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Lexical analysis
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****************
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.. index:: lexical analysis, parser, token
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A Python program is read by a *parser*. Input to the parser is a stream of
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*tokens*, generated by the *lexical analyzer*. This chapter describes how the
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lexical analyzer breaks a file into tokens.
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Python reads program text as Unicode code points; the encoding of a source file
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can be given by an encoding declaration and defaults to UTF-8, see :pep:`3120`
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for details. If the source file cannot be decoded, a :exc:`SyntaxError` is
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raised.
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.. _line-structure:
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Line structure
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==============
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.. index:: line structure
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A Python program is divided into a number of *logical lines*.
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.. _logical-lines:
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Logical lines
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-------------
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.. index:: logical line, physical line, line joining, NEWLINE token
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The end of a logical line is represented by the token NEWLINE. Statements
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cannot cross logical line boundaries except where NEWLINE is allowed by the
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syntax (e.g., between statements in compound statements). A logical line is
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constructed from one or more *physical lines* by following the explicit or
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implicit *line joining* rules.
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.. _physical-lines:
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Physical lines
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--------------
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A physical line is a sequence of characters terminated by an end-of-line
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sequence. In source files, any of the standard platform line termination
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sequences can be used - the Unix form using ASCII LF (linefeed), the Windows
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form using the ASCII sequence CR LF (return followed by linefeed), or the
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Macintosh form using the ASCII CR (return) character. All of these forms can be
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used equally, regardless of platform.
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When embedding Python, source code strings should be passed to Python APIs using
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the standard C conventions for newline characters (the ``\n`` character,
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representing ASCII LF, is the line terminator).
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.. _comments:
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Comments
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--------
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.. index:: comment, hash character
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A comment starts with a hash character (``#``) that is not part of a string
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literal, and ends at the end of the physical line. A comment signifies the end
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of the logical line unless the implicit line joining rules are invoked. Comments
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are ignored by the syntax; they are not tokens.
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.. _encodings:
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Encoding declarations
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---------------------
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.. index:: source character set, encodings
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If a comment in the first or second line of the Python script matches the
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regular expression ``coding[=:]\s*([-\w.]+)``, this comment is processed as an
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encoding declaration; the first group of this expression names the encoding of
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the source code file. The recommended forms of this expression are ::
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# -*- coding: <encoding-name> -*-
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which is recognized also by GNU Emacs, and ::
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# vim:fileencoding=<encoding-name>
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which is recognized by Bram Moolenaar's VIM.
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If no encoding declaration is found, the default encoding is UTF-8. In
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addition, if the first bytes of the file are the UTF-8 byte-order mark
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(``b'\xef\xbb\xbf'``), the declared file encoding is UTF-8 (this is supported,
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among others, by Microsoft's :program:`notepad`).
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If an encoding is declared, the encoding name must be recognized by Python. The
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encoding is used for all lexical analysis, including string literals, comments
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and identifiers. The encoding declaration must appear on a line of its own.
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.. XXX there should be a list of supported encodings.
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.. _explicit-joining:
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Explicit line joining
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---------------------
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.. index:: physical line, line joining, line continuation, backslash character
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Two or more physical lines may be joined into logical lines using backslash
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characters (``\``), as follows: when a physical line ends in a backslash that is
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not part of a string literal or comment, it is joined with the following forming
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a single logical line, deleting the backslash and the following end-of-line
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character. For example::
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if 1900 < year < 2100 and 1 <= month <= 12 \
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and 1 <= day <= 31 and 0 <= hour < 24 \
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and 0 <= minute < 60 and 0 <= second < 60: # Looks like a valid date
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return 1
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A line ending in a backslash cannot carry a comment. A backslash does not
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continue a comment. A backslash does not continue a token except for string
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literals (i.e., tokens other than string literals cannot be split across
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physical lines using a backslash). A backslash is illegal elsewhere on a line
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outside a string literal.
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.. _implicit-joining:
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Implicit line joining
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---------------------
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Expressions in parentheses, square brackets or curly braces can be split over
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more than one physical line without using backslashes. For example::
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month_names = ['Januari', 'Februari', 'Maart', # These are the
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'April', 'Mei', 'Juni', # Dutch names
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'Juli', 'Augustus', 'September', # for the months
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'Oktober', 'November', 'December'] # of the year
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Implicitly continued lines can carry comments. The indentation of the
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continuation lines is not important. Blank continuation lines are allowed.
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There is no NEWLINE token between implicit continuation lines. Implicitly
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continued lines can also occur within triple-quoted strings (see below); in that
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case they cannot carry comments.
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.. _blank-lines:
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Blank lines
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-----------
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.. index:: single: blank line
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A logical line that contains only spaces, tabs, formfeeds and possibly a
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comment, is ignored (i.e., no NEWLINE token is generated). During interactive
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input of statements, handling of a blank line may differ depending on the
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implementation of the read-eval-print loop. In the standard interactive
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interpreter, an entirely blank logical line (i.e. one containing not even
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whitespace or a comment) terminates a multi-line statement.
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.. _indentation:
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Indentation
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-----------
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.. index:: indentation, leading whitespace, space, tab, grouping, statement grouping
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Leading whitespace (spaces and tabs) at the beginning of a logical line is used
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to compute the indentation level of the line, which in turn is used to determine
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the grouping of statements.
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First, tabs are replaced (from left to right) by one to eight spaces such that
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the total number of characters up to and including the replacement is a multiple
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of eight (this is intended to be the same rule as used by Unix). The total
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number of spaces preceding the first non-blank character then determines the
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line's indentation. Indentation cannot be split over multiple physical lines
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using backslashes; the whitespace up to the first backslash determines the
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indentation.
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**Cross-platform compatibility note:** because of the nature of text editors on
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non-UNIX platforms, it is unwise to use a mixture of spaces and tabs for the
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indentation in a single source file. It should also be noted that different
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platforms may explicitly limit the maximum indentation level.
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A formfeed character may be present at the start of the line; it will be ignored
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for the indentation calculations above. Formfeed characters occurring elsewhere
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in the leading whitespace have an undefined effect (for instance, they may reset
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the space count to zero).
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.. index:: INDENT token, DEDENT token
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The indentation levels of consecutive lines are used to generate INDENT and
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DEDENT tokens, using a stack, as follows.
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Before the first line of the file is read, a single zero is pushed on the stack;
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this will never be popped off again. The numbers pushed on the stack will
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always be strictly increasing from bottom to top. At the beginning of each
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logical line, the line's indentation level is compared to the top of the stack.
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If it is equal, nothing happens. If it is larger, it is pushed on the stack, and
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one INDENT token is generated. If it is smaller, it *must* be one of the
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numbers occurring on the stack; all numbers on the stack that are larger are
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popped off, and for each number popped off a DEDENT token is generated. At the
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end of the file, a DEDENT token is generated for each number remaining on the
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stack that is larger than zero.
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Here is an example of a correctly (though confusingly) indented piece of Python
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code::
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def perm(l):
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# Compute the list of all permutations of l
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if len(l) <= 1:
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return [l]
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r = []
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for i in range(len(l)):
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s = l[:i] + l[i+1:]
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p = perm(s)
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for x in p:
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r.append(l[i:i+1] + x)
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return r
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The following example shows various indentation errors::
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def perm(l): # error: first line indented
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for i in range(len(l)): # error: not indented
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s = l[:i] + l[i+1:]
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p = perm(l[:i] + l[i+1:]) # error: unexpected indent
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for x in p:
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r.append(l[i:i+1] + x)
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return r # error: inconsistent dedent
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(Actually, the first three errors are detected by the parser; only the last
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error is found by the lexical analyzer --- the indentation of ``return r`` does
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not match a level popped off the stack.)
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.. _whitespace:
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Whitespace between tokens
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-------------------------
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Except at the beginning of a logical line or in string literals, the whitespace
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characters space, tab and formfeed can be used interchangeably to separate
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tokens. Whitespace is needed between two tokens only if their concatenation
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could otherwise be interpreted as a different token (e.g., ab is one token, but
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a b is two tokens).
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.. _other-tokens:
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Other tokens
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============
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Besides NEWLINE, INDENT and DEDENT, the following categories of tokens exist:
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*identifiers*, *keywords*, *literals*, *operators*, and *delimiters*. Whitespace
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characters (other than line terminators, discussed earlier) are not tokens, but
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serve to delimit tokens. Where ambiguity exists, a token comprises the longest
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possible string that forms a legal token, when read from left to right.
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.. _identifiers:
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Identifiers and keywords
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========================
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.. index:: identifier, name
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Identifiers (also referred to as *names*) are described by the following lexical
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definitions:
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The syntax of identifiers in Python is based on the Unicode standard annex
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UAX-31, with elaboration and changes as defined below.
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Within the ASCII range (U+0001..U+007F), the valid characters for identifiers
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are the same as in Python 2.5; Python 3.0 introduces additional
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characters from outside the ASCII range (see :pep:`3131`). For other
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characters, the classification uses the version of the Unicode Character
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Database as included in the :mod:`unicodedata` module.
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Identifiers are unlimited in length. Case is significant.
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.. productionlist::
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identifier: `id_start` `id_continue`*
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id_start: <all characters in general categories Lu, Ll, Lt, Lm, Lo, Nl, the underscore, and characters with the Other_ID_Start property>
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id_continue: <all characters in `id_start`, plus characters in the categories Mn, Mc, Nd, Pc and others with the Other_ID_Continue property>
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The Unicode category codes mentioned above stand for:
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* *Lu* - uppercase letters
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* *Ll* - lowercase letters
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* *Lt* - titlecase letters
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* *Lm* - modifier letters
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* *Lo* - other letters
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* *Nl* - letter numbers
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* *Mn* - nonspacing marks
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* *Mc* - spacing combining marks
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* *Nd* - decimal numbers
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* *Pc* - connector punctuations
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All identifiers are converted into the normal form NFC while parsing; comparison
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of identifiers is based on NFC.
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A non-normative HTML file listing all valid identifier characters for Unicode
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4.1 can be found at
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http://www.dcl.hpi.uni-potsdam.de/home/loewis/table-3131.html.
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See :pep:`3131` for further details.
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.. _keywords:
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Keywords
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--------
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.. index::
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single: keyword
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single: reserved word
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The following identifiers are used as reserved words, or *keywords* of the
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language, and cannot be used as ordinary identifiers. They must be spelled
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exactly as written here::
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False class finally is return
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None continue for lambda try
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True def from nonlocal while
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and del global not with
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as elif if or yield
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assert else import pass
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break except in raise
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.. _id-classes:
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Reserved classes of identifiers
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-------------------------------
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Certain classes of identifiers (besides keywords) have special meanings. These
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classes are identified by the patterns of leading and trailing underscore
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characters:
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``_*``
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Not imported by ``from module import *``. The special identifier ``_`` is used
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in the interactive interpreter to store the result of the last evaluation; it is
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stored in the :mod:`builtins` module. When not in interactive mode, ``_``
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has no special meaning and is not defined. See section :ref:`import`.
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.. note::
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The name ``_`` is often used in conjunction with internationalization;
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refer to the documentation for the :mod:`gettext` module for more
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information on this convention.
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``__*__``
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System-defined names. These names are defined by the interpreter and its
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implementation (including the standard library); applications should not expect
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to define additional names using this convention. The set of names of this
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class defined by Python may be extended in future versions. See section
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:ref:`specialnames`.
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``__*``
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Class-private names. Names in this category, when used within the context of a
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class definition, are re-written to use a mangled form to help avoid name
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clashes between "private" attributes of base and derived classes. See section
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:ref:`atom-identifiers`.
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.. _literals:
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Literals
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========
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.. index:: literal, constant
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Literals are notations for constant values of some built-in types.
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.. _strings:
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String and Bytes literals
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-------------------------
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.. index:: string literal, bytes literal, ASCII
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String literals are described by the following lexical definitions:
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.. productionlist::
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stringliteral: [`stringprefix`](`shortstring` | `longstring`)
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stringprefix: "r" | "R"
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shortstring: "'" `shortstringitem`* "'" | '"' `shortstringitem`* '"'
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longstring: "'''" `longstringitem`* "'''" | '"""' `longstringitem`* '"""'
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shortstringitem: `shortstringchar` | `stringescapeseq`
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longstringitem: `longstringchar` | `stringescapeseq`
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shortstringchar: <any source character except "\" or newline or the quote>
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longstringchar: <any source character except "\">
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stringescapeseq: "\" <any source character>
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.. productionlist::
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bytesliteral: `bytesprefix`(`shortbytes` | `longbytes`)
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bytesprefix: "b" | "B"
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shortbytes: "'" `shortbytesitem`* "'" | '"' `shortbytesitem`* '"'
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longbytes: "'''" `longbytesitem`* "'''" | '"""' `longbytesitem`* '"""'
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shortbytesitem: `shortbyteschar` | `bytesescapeseq`
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longbytesitem: `longbyteschar` | `bytesescapeseq`
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shortbyteschar: <any ASCII character except "\" or newline or the quote>
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longbyteschar: <any ASCII character except "\">
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bytesescapeseq: "\" <any ASCII character>
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One syntactic restriction not indicated by these productions is that whitespace
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is not allowed between the :token:`stringprefix` or :token:`bytesprefix` and the
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rest of the literal. The source character set is defined by the encoding
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declaration; it is UTF-8 if no encoding declaration is given in the source file;
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see section :ref:`encodings`.
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.. index:: triple-quoted string, Unicode Consortium, raw string
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In plain English: Both types of literals can be enclosed in matching single quotes
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(``'``) or double quotes (``"``). They can also be enclosed in matching groups
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of three single or double quotes (these are generally referred to as
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*triple-quoted strings*). The backslash (``\``) character is used to escape
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characters that otherwise have a special meaning, such as newline, backslash
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itself, or the quote character.
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String literals may optionally be prefixed with a letter ``'r'`` or ``'R'``;
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such strings are called :dfn:`raw strings` and use different rules for
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interpreting backslash escape sequences.
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Bytes literals are always prefixed with ``'b'`` or ``'B'``; they produce an
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instance of the :class:`bytes` type instead of the :class:`str` type. They
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may only contain ASCII characters; bytes with a numeric value of 128 or greater
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must be expressed with escapes.
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In triple-quoted strings, unescaped newlines and quotes are allowed (and are
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retained), except that three unescaped quotes in a row terminate the string. (A
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"quote" is the character used to open the string, i.e. either ``'`` or ``"``.)
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.. index:: physical line, escape sequence, Standard C, C
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Unless an ``'r'`` or ``'R'`` prefix is present, escape sequences in strings are
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interpreted according to rules similar to those used by Standard C. The
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recognized escape sequences are:
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+-----------------+---------------------------------+-------+
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| Escape Sequence | Meaning | Notes |
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+=================+=================================+=======+
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| ``\newline`` | Backslash and newline ignored | |
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+-----------------+---------------------------------+-------+
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| ``\\`` | Backslash (``\``) | |
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+-----------------+---------------------------------+-------+
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| ``\'`` | Single quote (``'``) | |
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+-----------------+---------------------------------+-------+
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| ``\"`` | Double quote (``"``) | |
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+-----------------+---------------------------------+-------+
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| ``\a`` | ASCII Bell (BEL) | |
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+-----------------+---------------------------------+-------+
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| ``\b`` | ASCII Backspace (BS) | |
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+-----------------+---------------------------------+-------+
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| ``\f`` | ASCII Formfeed (FF) | |
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+-----------------+---------------------------------+-------+
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| ``\n`` | ASCII Linefeed (LF) | |
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+-----------------+---------------------------------+-------+
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| ``\r`` | ASCII Carriage Return (CR) | |
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+-----------------+---------------------------------+-------+
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| ``\t`` | ASCII Horizontal Tab (TAB) | |
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+-----------------+---------------------------------+-------+
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| ``\v`` | ASCII Vertical Tab (VT) | |
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+-----------------+---------------------------------+-------+
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| ``\ooo`` | Character with octal value | (1,3) |
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| | *ooo* | |
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+-----------------+---------------------------------+-------+
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| ``\xhh`` | Character with hex value *hh* | (2,3) |
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+-----------------+---------------------------------+-------+
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Escape sequences only recognized in string literals are:
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+-----------------+---------------------------------+-------+
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| Escape Sequence | Meaning | Notes |
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+=================+=================================+=======+
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| ``\N{name}`` | Character named *name* in the | |
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| | Unicode database | |
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+-----------------+---------------------------------+-------+
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| ``\uxxxx`` | Character with 16-bit hex value | \(4) |
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| | *xxxx* | |
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+-----------------+---------------------------------+-------+
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| ``\Uxxxxxxxx`` | Character with 32-bit hex value | \(5) |
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| | *xxxxxxxx* | |
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+-----------------+---------------------------------+-------+
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Notes:
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(1)
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As in Standard C, up to three octal digits are accepted.
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(2)
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Unlike in Standard C, at most two hex digits are accepted.
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(3)
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In a bytes literal, hexadecimal and octal escapes denote the byte with the
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given value. In a string literal, these escapes denote a Unicode character
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with the given value.
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(4)
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Individual code units which form parts of a surrogate pair can be encoded using
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this escape sequence.
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(5)
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Any Unicode character can be encoded this way, but characters outside the Basic
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Multilingual Plane (BMP) will be encoded using a surrogate pair if Python is
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compiled to use 16-bit code units (the default). Individual code units which
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form parts of a surrogate pair can be encoded using this escape sequence.
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.. index:: unrecognized escape sequence
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Unlike Standard C, all unrecognized escape sequences are left in the string
|
|
unchanged, i.e., *the backslash is left in the string*. (This behavior is
|
|
useful when debugging: if an escape sequence is mistyped, the resulting output
|
|
is more easily recognized as broken.) It is also important to note that the
|
|
escape sequences only recognized in string literals fall into the category of
|
|
unrecognized escapes for bytes literals.
|
|
|
|
When an ``'r'`` or ``'R'`` prefix is used in a string literal, then the
|
|
``\uXXXX`` and ``\UXXXXXXXX`` escape sequences are processed while *all other
|
|
backslashes are left in the string*. For example, the string literal
|
|
``r"\u0062\n"`` consists of three Unicode characters: 'LATIN SMALL LETTER B',
|
|
'REVERSE SOLIDUS', and 'LATIN SMALL LETTER N'. Backslashes can be escaped with a
|
|
preceding backslash; however, both remain in the string. As a result,
|
|
``\uXXXX`` escape sequences are only recognized when there is an odd number of
|
|
backslashes.
|
|
|
|
Even in a raw string, string quotes can be escaped with a backslash, but the
|
|
backslash remains in the string; for example, ``r"\""`` is a valid string
|
|
literal 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 of
|
|
backslashes). Specifically, *a raw string cannot end in a single backslash*
|
|
(since the backslash would escape the following quote character). Note also
|
|
that a single backslash followed by a newline is interpreted as those two
|
|
characters as part of the string, *not* as a line continuation.
|
|
|
|
|
|
.. _string-catenation:
|
|
|
|
String literal concatenation
|
|
----------------------------
|
|
|
|
Multiple adjacent string literals (delimited by whitespace), possibly using
|
|
different quoting conventions, are allowed, and their meaning is the same as
|
|
their concatenation. Thus, ``"hello" 'world'`` is equivalent to
|
|
``"helloworld"``. This feature can be used to reduce the number of backslashes
|
|
needed, to split long strings conveniently across long lines, or even to add
|
|
comments 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 at
|
|
compile time. The '+' operator must be used to concatenate string expressions
|
|
at run time. Also note that literal concatenation can use different quoting
|
|
styles for each component (even mixing raw strings and triple quoted strings).
|
|
|
|
|
|
.. _numbers:
|
|
|
|
Numeric literals
|
|
----------------
|
|
|
|
.. index:: number, numeric literal, integer literal
|
|
floating point literal, hexadecimal literal
|
|
octal literal, binary literal, decimal literal, imaginary literal, complex literal
|
|
|
|
There are three types of numeric literals: plain integers, floating point
|
|
numbers, and imaginary numbers. There are no complex literals
|
|
(complex numbers can be formed by adding a real number and an imaginary number).
|
|
|
|
Note that numeric literals do not include a sign; a phrase like ``-1`` is
|
|
actually an expression composed of the unary operator '``-``' and the literal
|
|
``1``.
|
|
|
|
|
|
.. _integers:
|
|
|
|
Integer literals
|
|
----------------
|
|
|
|
Integer literals are described by the following lexical definitions:
|
|
|
|
.. productionlist::
|
|
integer: `decimalinteger` | `octinteger` | `hexinteger`
|
|
decimalinteger: `nonzerodigit` `digit`* | "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 be
|
|
stored in available memory.
|
|
|
|
Note that leading zeros in a non-zero decimal number are not allowed. This is
|
|
for disambiguation with C-style octal literals, which Python used before version
|
|
3.0.
|
|
|
|
Some examples of integer literals::
|
|
|
|
7 2147483647 0o177 0b100110111
|
|
3 79228162514264337593543950336 0o377 0x100000000
|
|
79228162514264337593543950336 0xdeadbeef
|
|
|
|
|
|
.. _floating:
|
|
|
|
Floating point literals
|
|
-----------------------
|
|
|
|
Floating point literals are described by the following lexical definitions:
|
|
|
|
.. productionlist::
|
|
floatnumber: `pointfloat` | `exponentfloat`
|
|
pointfloat: [`intpart`] `fraction` | `intpart` "."
|
|
exponentfloat: (`intpart` | `pointfloat`) `exponent`
|
|
intpart: `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 as ``77e10``. The
|
|
allowed range of floating point literals is implementation-dependent. Some
|
|
examples of floating point literals::
|
|
|
|
3.14 10. .001 1e100 3.14e-10 0e0
|
|
|
|
Note that numeric literals do not include a sign; a phrase like ``-1`` is
|
|
actually an expression composed of the unary operator ``-`` and the literal
|
|
``1``.
|
|
|
|
|
|
.. _imaginary:
|
|
|
|
Imaginary literals
|
|
------------------
|
|
|
|
Imaginary literals are described by the following lexical definitions:
|
|
|
|
.. productionlist::
|
|
imagnumber: (`floatnumber` | `intpart`) ("j" | "J")
|
|
|
|
An imaginary literal yields a complex number with a real part of 0.0. Complex
|
|
numbers are represented as a pair of floating point numbers and have the same
|
|
restrictions on their range. To create a complex number with a nonzero real
|
|
part, add a floating point number to it, e.g., ``(3+4j)``. Some examples of
|
|
imaginary literals::
|
|
|
|
3.14j 10.j 10j .001j 1e100j 3.14e-10j
|
|
|
|
|
|
.. _operators:
|
|
|
|
Operators
|
|
=========
|
|
|
|
.. index:: single: operators
|
|
|
|
The following tokens are operators::
|
|
|
|
+ - * ** / // %
|
|
<< >> & | ^ ~
|
|
< > <= >= == !=
|
|
|
|
|
|
.. _delimiters:
|
|
|
|
Delimiters
|
|
==========
|
|
|
|
.. index:: single: delimiters
|
|
|
|
The following tokens serve as delimiters in the grammar::
|
|
|
|
( ) [ ] { } @
|
|
, : . ` = ;
|
|
+= -= *= /= //= %=
|
|
&= |= ^= >>= <<= **=
|
|
|
|
The period can also occur in floating-point and imaginary literals. A sequence
|
|
of three periods has a special meaning as an ellipsis literal. The second half
|
|
of the list, the augmented assignment operators, serve lexically as delimiters,
|
|
but also perform an operation.
|
|
|
|
The following printing ASCII characters have special meaning as part of other
|
|
tokens or are otherwise significant to the lexical analyzer::
|
|
|
|
' " # \
|
|
|
|
The following printing ASCII characters are not used in Python. Their
|
|
occurrence outside string literals and comments is an unconditional error::
|
|
|
|
$ ?
|