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" identifier: <NAME, except keywords>
Note that not all names matched by this grammar are valid; see 名稱中的非 ASCII 字元 for details.
2.3.1. 關鍵字¶
The following names are used as reserved words, or keywords of the language, and cannot be used as ordinary identifiers. They must be spelled exactly as written here:
False await else import pass
None break except in raise
True class finally is return
and continue for lambda try
as def from nonlocal while
assert del global not with
async elif if or yield
2.3.2. 軟關鍵字¶
在 3.10 版被加入.
Some names are only reserved under specific contexts. These are known as soft keywords:
These syntactically act as keywords in their specific contexts, but this distinction is done at the parser level, not when tokenizing.
As soft keywords, their use in the grammar is possible while still preserving compatibility with existing code that uses these names as identifier names.
在 3.12 版的變更: type 現在是軟關鍵字。
2.3.3. Reserved classes of identifiers¶
Certain classes of identifiers (besides keywords) have special meanings. These classes are identified by the patterns of leading and trailing underscore characters:
_*Not imported by
from module import *._In a
casepattern within amatchstatement,_is a soft keyword that denotes a wildcard.Separately, the interactive interpreter makes the result of the last evaluation available in the variable
_. (It is stored in thebuiltinsmodule, alongside built-in functions likeprint.)Elsewhere,
_is a regular identifier. It is often used to name "special" items, but it is not special to Python itself.備註
The name
_is often used in conjunction with internationalization; refer to the documentation for thegettextmodule for more information on this convention.It is also commonly used for unused variables.
__*__System-defined names, informally known as "dunder" names. These names are defined by the interpreter and its implementation (including the standard library). Current system names are discussed in the Special method names section and elsewhere. More will likely be defined in future versions of Python. Any use of
__*__names, in any context, that does not follow explicitly documented use, is subject to breakage without warning.__*Class-private names. Names in this category, when used within the context of a class definition, are re-written to use a mangled form to help avoid name clashes between "private" attributes of base and derived classes. See section Identifiers (Names).
2.3.4. 名稱中的非 ASCII 字元¶
Names that contain non-ASCII characters need additional normalization
and validation beyond the rules and grammar explained
above.
For example, ř_1, 蛇, or साँप are valid names, but r〰2,
€, or 🐍 are not.
This section explains the exact rules.
All names are converted into the normalization form NFKC while parsing.
This means that, for example, some typographic variants of characters are
converted to their "basic" form. For example, fiⁿₐˡᵢᶻₐᵗᵢᵒₙ normalizes to
finalization, so Python treats them as the same name:
>>> fiⁿₐˡᵢᶻₐᵗᵢᵒₙ = 3
>>> finalization
3
備註
Normalization is done at the lexical level only.
Run-time functions that take names as strings generally do not normalize
their arguments.
For example, the variable defined above is accessible at run time in the
globals() dictionary as globals()["finalization"] but not
globals()["fiⁿₐˡᵢᶻₐᵗᵢᵒₙ"].
Similarly to how ASCII-only names must contain only letters, digits and
the underscore, and cannot start with a digit, a valid name must
start with a character in the "letter-like" set xid_start,
and the remaining characters must be in the "letter- and digit-like" set
xid_continue.
These sets are based on the XID_Start and XID_Continue sets as defined by the
Unicode standard annex UAX-31.
Python's xid_start additionally includes the underscore (_).
Note that Python does not necessarily conform to UAX-31.
A non-normative listing of characters in the XID_Start and XID_Continue
sets as defined by Unicode is available in the DerivedCoreProperties.txt
file in the Unicode Character Database.
For reference, the construction rules for the xid_* sets are given below.
The set id_start is defined as the union of:
Unicode category
<Lu>- uppercase letters (includesAtoZ)Unicode category
<Ll>- lowercase letters (includesatoz)Unicode category
<Lt>- titlecase lettersUnicode category
<Lm>- modifier lettersUnicode category
<Lo>- other lettersUnicode category
<Nl>- letter numbers{
"_"} - 底線<Other_ID_Start>- an explicit set of characters in PropList.txt to support backwards compatibility
The set xid_start then closes this set under NFKC normalization, by
removing all characters whose normalization is not of the form
id_start id_continue*.
The set id_continue is defined as the union of:
id_start(see above)Unicode category
<Nd>- decimal numbers (includes0to9)Unicode category
<Pc>- connector punctuationsUnicode category
<Mn>- nonspacing marksUnicode category
<Mc>- spacing combining marks<Other_ID_Continue>- another explicit set of characters in PropList.txt to support backwards compatibility
Again, xid_continue closes this set under NFKC normalization.
Unicode categories use the version of the Unicode Character Database as
included in the unicodedata module.
2.4. Literals¶
Literals are notations for constant values of some built-in types.
In terms of lexical analysis, Python has string, bytes and numeric literals.
Other "literals" are lexically denoted using keywords
(None, True, False) and the special
ellipsis token (...).
2.5. String and Bytes literals¶
String literals are text enclosed in single quotes (') or double
quotes ("). For example:
"spam"
'eggs'
The quote used to start the literal also terminates it, so a string literal can only contain the other quote (except with escape sequences, see below). For example:
'Say "Hello", please.'
"Don't do that!"
Except for this limitation, the choice of quote character (' or ")
does not affect how the literal is parsed.
Inside a string literal, the backslash (\) character introduces an
escape sequence, which has special meaning depending on the character
after the backslash.
For example, \" denotes the double quote character, and does not end
the string:
>>> print("Say \"Hello\" to everyone!")
Say "Hello" to everyone!
See escape sequences below for a full list of such sequences, and more details.
2.5.1. 三引號字串¶
Strings can also be enclosed in matching groups of three single or double quotes. These are generally referred to as triple-quoted strings:
"""這是一個三引號字串。"""
In triple-quoted literals, unescaped quotes are allowed (and are
retained), except that three unescaped quotes in a row terminate the literal,
if they are of the same kind (' or ") used at the start:
"""這個字串裡面有 "引號"。"""
Unescaped newlines are also allowed and retained:
'''This triple-quoted string
continues on the next line.'''
2.5.2. String prefixes¶
String literals can have an optional prefix that influences how the content of the literal is parsed, for example:
b"data"
f'{result=}'
The allowed prefixes are:
r: Raw string
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.