2. 詞法分析¶
A Python program is read by a parser. Input to the parser is a stream of tokens, generated by the lexical analyzer (also known as the tokenizer). This chapter describes how the lexical analyzer produces these tokens.
The lexical analyzer determines the program text's encoding
(UTF-8 by default), and decodes the text into
source characters.
If the text cannot be decoded, a SyntaxError is raised.
Next, the lexical analyzer uses the source characters to generate a stream of tokens. The type of a generated token generally depends on the next source character to be processed. Similarly, other special behavior of the analyzer depends on the first source character that hasn't yet been processed. The following table gives a quick summary of these source characters, with links to sections that contain more information.
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2.1. Line structure¶
A Python program is divided into a number of logical lines.
2.1.1. Logical lines¶
The end of a logical line is represented by the token NEWLINE.
Statements cannot cross logical line boundaries except where NEWLINE
is allowed by the syntax (e.g., between statements in compound statements).
A logical line is constructed from one or more physical lines by following
the explicit or implicit
line joining rules.
2.1.2. Physical lines¶
A physical line is a sequence of characters terminated by one the following end-of-line sequences:
the Unix form using ASCII LF (linefeed),
the Windows form using the ASCII sequence CR LF (return followed by linefeed),
the 'Classic Mac OS' form using the ASCII CR (return) character.
Regardless of platform, each of these sequences is replaced by a single ASCII LF (linefeed) character. (This is done even inside string literals.) Each line can use any of the sequences; they do not need to be consistent within a file.
The end of input also serves as an implicit terminator for the final physical line.
Formally:
newline: <ASCII LF> | <ASCII CR> <ASCII LF> | <ASCII CR>
2.1.4. Encoding declarations¶
If a comment in the first or second line of the Python script matches the
regular expression coding[=:]\s*([-\w.]+), this comment is processed as an
encoding declaration; the first group of this expression names the encoding of
the source code file. The encoding declaration must appear on a line of its
own. If it is the second line, the first line must also be a comment-only line.
The recommended forms of an encoding expression are
# -*- coding: <encoding-name> -*-
which is recognized also by GNU Emacs, and
# vim:fileencoding=<encoding-name>
which is recognized by Bram Moolenaar's VIM.
If no encoding declaration is found, the default encoding is UTF-8. If the
implicit or explicit encoding of a file is UTF-8, an initial UTF-8 byte-order
mark (b'\xef\xbb\xbf') is ignored rather than being a syntax error.
If an encoding is declared, the encoding name must be recognized by Python (see 標準編碼). The encoding is used for all lexical analysis, including string literals, comments and identifiers.
All lexical analysis, including string literals, comments and identifiers, works on Unicode text decoded using the source encoding. Any Unicode code point, except the NUL control character, can appear in Python source.
source_character: <any Unicode code point, except NUL>
2.1.5. Explicit line joining¶
Two or more physical lines may be joined into logical lines using backslash
characters (\), as follows: when a physical line ends in a backslash that is
not part of a string literal or comment, it is joined with the following forming
a single logical line, deleting the backslash and the following end-of-line
character. For example:
if 1900 < year < 2100 and 1 <= month <= 12 \
and 1 <= day <= 31 and 0 <= hour < 24 \
and 0 <= minute < 60 and 0 <= second < 60: # 看起來像個有效日期
return 1
A line ending in a backslash cannot carry a comment. A backslash does not continue a comment. A backslash does not continue a token except for string literals (i.e., tokens other than string literals cannot be split across physical lines using a backslash). A backslash is illegal elsewhere on a line outside a string literal.
2.1.6. Implicit line joining¶
Expressions in parentheses, square brackets or curly braces can be split over more than one physical line without using backslashes. For example:
month_names = ['Januari', 'Februari', 'Maart', # 這些是
'April', 'Mei', 'Juni', # 荷蘭文的
'Juli', 'Augustus', 'September', # 一年之中
'Oktober', 'November', 'December'] # 月份名稱
Implicitly continued lines can carry comments. The indentation of the continuation lines is not important. Blank continuation lines are allowed. There is no NEWLINE token between implicit continuation lines. Implicitly continued lines can also occur within triple-quoted strings (see below); in that case they cannot carry comments.
2.1.7. Blank lines¶
A logical line that contains only spaces, tabs, formfeeds and possibly a
comment, is ignored (i.e., no NEWLINE token is generated).
During interactive input of statements, handling of a blank line may differ
depending on the implementation of the read-eval-print loop.
In the standard interactive interpreter, an entirely blank logical line (that
is, one containing not even whitespace 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 used to compute the indentation level of the line, which in turn is used to determine the grouping of statements.
Tabs are replaced (from left to right) by one to eight spaces such that the total number of characters up to and including the replacement is a multiple of eight (this is intended to be the same rule as used by Unix). The total number of spaces preceding the first non-blank character then determines the line's indentation. Indentation cannot be split over multiple physical lines using backslashes; the whitespace up to the first backslash determines the indentation.
Indentation is rejected as inconsistent if a source file mixes tabs and spaces
in a way that makes the meaning dependent on the worth of a tab in spaces; a
TabError is raised in that case.
Cross-platform compatibility note: because of the nature of text editors on non-UNIX platforms, it is unwise to use a mixture of spaces and tabs for the indentation in a single source file. It should also be noted that different platforms may explicitly limit the maximum indentation level.
A formfeed character may be present at the start of the line; it will be ignored for the indentation calculations above. Formfeed characters occurring elsewhere in the leading whitespace have an undefined effect (for instance, they may reset the space count to zero).
The indentation levels of consecutive lines are used to generate
INDENT and DEDENT 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 will
always be strictly increasing from bottom to top. At the beginning of each
logical 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, and
one INDENT token is generated. If it is smaller, it must be one of the
numbers occurring on the stack; all numbers on the stack that are larger are
popped off, and for each number popped off a DEDENT token is generated.
At the end of the file, a DEDENT token is generated for each number
remaining on the stack that is larger than zero.
Here is an example of a correctly (though confusingly) indented piece of Python code:
def perm(l):
# Compute the list of all permutations of l
if len(l) <= 1:
return [l]
r = []
for i in range(len(l)):
s = l[:i] + l[i+1:]
p = perm(s)
for x in p:
r.append(l[i:i+1] + x)
return r
The following example shows various indentation errors:
def perm(l): # error: first line indented
for i in range(len(l)): # error: not indented
s = l[:i] + l[i+1:]
p = perm(l[:i] + l[i+1:]) # error: unexpected indent
for x in p:
r.append(l[i:i+1] + x)
return r # error: inconsistent dedent
(Actually, the first three errors are detected by the parser; only the last
error is found by the lexical analyzer --- the indentation of return r does
not 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 whitespace characters space, tab and formfeed can be used interchangeably to separate tokens:
whitespace: ' ' | tab | formfeed
Whitespace is needed between two tokens only if their concatenation
could otherwise be interpreted as a different token. For example, ab is one
token, but a b is two tokens. However, +a and + a both produce
two tokens, + and a, as +a is not a valid token.
2.1.10. End marker¶
At the end of non-interactive input, the lexical analyzer generates an
ENDMARKER token.
2.2. Other tokens¶
Besides NEWLINE, INDENT and DEDENT,
the following categories of tokens exist:
identifiers and keywords (NAME), literals (such as
NUMBER and STRING), and other symbols
(operators and delimiters, OP).
Whitespace characters (other than logical line terminators, discussed earlier)
are not tokens, but serve to delimit tokens.
Where ambiguity exists, a token comprises the longest possible string that
forms a legal token, when read from left to right.
2.3. Names (identifiers and keywords)¶
NAME tokens represent identifiers, keywords, and
soft keywords.
Names are composed of the following characters:
uppercase and lowercase letters (
A-Zanda-z),底線 (
_)，digits (
0through9), which cannot appear as the first character, andnon-ASCII characters. Valid names may only contain "letter-like" and "digit-like" characters; see 名稱中的非 ASCII 字元 for details.
Names must contain at least one character, but have no upper length limit. Case is significant.
Formally, names are described by the following lexical definitions:
NAME:name_startname_continue* name_start: "a"..."z" | "A"..."Z" | "_" | <non-ASCII character> name_continue: name_start | "0"..."9"
2.1.3. Comments¶
A comment starts with a hash character (
#) that is not part of a string literal, and ends at the end of the physical line. A comment signifies the end of the logical line unless the implicit line joining rules are invoked. Comments are ignored by the syntax.