一些铺垫（扯淡）
历史上，在Python 2.4以及之前的版本，py代码的执行，也就是从源码到bytecode分为两步：
解析py源码成为分析树 (Parser/pgen.c)
基于分析树优化缩减bytecode (Python/compile.c)
也是由于Guido van Rossum大叔的历史局限性，Python当年就被实现成这么一个样子了，运行的也挺好。后来随着LLVM党的崛起，大家逐渐认识到这种模式的局限性，后面还会说。
正经说起来，一个标准的“现代”解释器（编译器）应该是“介事儿”的：
把py源码解析成分析树 (Parser/pgen.c)
把分析树转化成抽象语法树AST（Abstract Syntax Tree） (Python/ast.c)
把AST转化成CFG（Control Flow Graph） (Python/compile.c)
基于Control Flow Graph优化生成bytecode (Python/compile.c)
从Python 2.5开始，CPython“改邪归正”走上了现代编译器的康庄大道。简单来说就是把原先的第二步拆解成3个步骤。后面我们会着重简要讲一下后面三步的过程：
这里不会阐述太多关于语法解析如何工作的内容，除了必须的有助于我们了解编译过程的内容。如果想要了解Python语法解析的细节，还是建议你去直接阅读CPython的源码。
语法分析树
2.5版以后的Python的语法解释器是基于“龙书”的LL(1) parser的一个标准的实现。Python的语法定义可以在CPython源码的Grammar/Grammar （https://github.com/python/cpython/blob/master/Grammar/Grammar）看到：
# Grammar for Python# NOTE WELL: You should also follow all the steps listed at# https://docs.python.org/devguide/grammar.html# Start symbols for the grammar:# single_input is a single interactive statement;# file_input is a module or sequence of commands read from an input file;# eval_input is the input for the eval() functions.# NB: compound_stmt in single_input is followed by extra NEWLINE!single_input: NEWLINE | simple_stmt | compound_stmt NEWLINE
file_input: (NEWLINE | stmt)* ENDMARKER
eval_input: testlist NEWLINE* ENDMARKER
decorator: '@' dotted_name [ '(' [arglist] ')' ] NEWLINE
decorators: decorator+
decorated: decorators (classdef | funcdef | async_funcdef)async_funcdef: ASYNC funcdef
funcdef: 'def' NAME parameters ['->' test] ':' suite
parameters: '(' [typedargslist] ')'typedargslist: (tfpdef ['=' test] (',' tfpdef ['=' test])* [',' [
'*' [tfpdef] (',' tfpdef ['=' test])* [',' ['**' tfpdef [',']]]
| '**' tfpdef [',']]]
| '*' [tfpdef] (',' tfpdef ['=' test])* [',' ['**' tfpdef [',']]]
| '**' tfpdef [','])tfpdef: NAME [':' test]varargslist: (vfpdef ['=' test] (',' vfpdef ['=' test])* [',' [
'*' [vfpdef] (',' vfpdef ['=' test])* [',' ['**' vfpdef [',']]]
| '**' vfpdef [',']]]
| '*' [vfpdef] (',' vfpdef ['=' test])* [',' ['**' vfpdef [',']]]
| '**' vfpdef [','])vfpdef: NAME
stmt: simple_stmt | compound_stmt
simple_stmt: small_stmt (';' small_stmt)* [';'] NEWLINE
small_stmt: (expr_stmt | del_stmt | pass_stmt | flow_stmt |
import_stmt | global_stmt | nonlocal_stmt | assert_stmt)expr_stmt: testlist_star_expr (annassign | augassign (yield_expr|testlist) |
('=' (yield_expr|testlist_star_expr))*)annassign: ':' test ['=' test]testlist_star_expr: (test|star_expr) (',' (test|star_expr))* [',']augassign: ('+=' | '-=' | '*=' | '@=' | '/=' | '%=' | '&=' | '|=' | '^=' |
'<<=' | '>>=' | '**=' | '//=')# For normal and annotated assignments, additional restrictions enforced by the interpreterdel_stmt: 'del' exprlist
pass_stmt: 'pass'flow_stmt: break_stmt | continue_stmt | return_stmt | raise_stmt | yield_stmt
break_stmt: 'break'continue_stmt: 'continue'return_stmt: 'return' [testlist]yield_stmt: yield_expr
raise_stmt: 'raise' [test ['from' test]]import_stmt: import_name | import_from
import_name: 'import' dotted_as_names# note below: the ('.' | '...') is necessary because '...' is tokenized as ELLIPSISimport_from: ('from' (('.' | '...')* dotted_name | ('.' | '...')+)
'import' ('*' | '(' import_as_names ')' | import_as_names))import_as_name: NAME ['as' NAME]dotted_as_name: dotted_name ['as' NAME]import_as_names: import_as_name (',' import_as_name)* [',']dotted_as_names: dotted_as_name (',' dotted_as_name)*
dotted_name: NAME ('.' NAME)*
global_stmt: 'global' NAME (',' NAME)*
nonlocal_stmt: 'nonlocal' NAME (',' NAME)*
assert_stmt: 'assert' test [',' test]compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | with_stmt | funcdef | classdef | decorated | async_stmt
async_stmt: ASYNC (funcdef | with_stmt | for_stmt)if_stmt: 'if' test ':' suite ('elif' test ':' suite)* ['else' ':' suite]while_stmt: 'while' test ':' suite ['else' ':' suite]for_stmt: 'for' exprlist 'in' testlist ':' suite ['else' ':' suite]try_stmt: ('try' ':' suite ((except_clause ':' suite)+ ['else' ':' suite]
['finally' ':' suite] |
'finally' ':' suite))with_stmt: 'with' with_item (',' with_item)* ':' suite
with_item: test ['as' expr]# NB compile.c makes sure that the default except clause is lastexcept_clause: 'except' [test ['as' NAME]]suite: simple_stmt | NEWLINE INDENT stmt+ DEDENT
test: or_test ['if' or_test 'else' test] | lambdef
test_nocond: or_test | lambdef_nocond
lambdef: 'lambda' [varargslist] ':' testlambdef_nocond: 'lambda' [varargslist] ':' test_nocond
or_test: and_test ('or' and_test)*
and_test: not_test ('and' not_test)*
not_test: 'not' not_test | comparison
comparison: expr (comp_op expr)*# <> isn't actually a valid comparison operator in Python. It's here for the# sake of a __future__ import described in PEP 401 (which really works :-)comp_op: '<'|'>'|'=='|'>='|'<='|'<>'|'!='|'in'|'not' 'in'|'is'|'is' 'not'star_expr: '*' expr
expr: xor_expr ('|' xor_expr)*
xor_expr: and_expr ('^' and_expr)*
and_expr: shift_expr ('&' shift_expr)*
shift_expr: arith_expr (('<<'|'>>') arith_expr)*
arith_expr: term (('+'|'-') term)*
term: factor (('*'|'@'|'/'|'%'|'//') factor)*
factor: ('+'|'-'|'~') factor | power
power: atom_expr ['**' factor]atom_expr: [AWAIT] atom trailer*
atom: ('(' [yield_expr|testlist_comp] ')' |
'[' [testlist_comp] ']' |
'{' [dictorsetmaker] '}' |
NAME | NUMBER | STRING+ | '...' | 'None' | 'True' | 'False')testlist_comp: (test|star_expr) ( comp_for | (',' (test|star_expr))* [','] )trailer: '(' [arglist] ')' | '[' subscriptlist ']' | '.' NAME
subscriptlist: subscript (',' subscript)* [',']subscript: test | [test] ':' [test] [sliceop]sliceop: ':' [test]exprlist: (expr|star_expr) (',' (expr|star_expr))* [',']testlist: test (',' test)* [',']dictorsetmaker: ( ((test ':' test | '**' expr)
(comp_for | (',' (test ':' test | '**' expr))* [','])) |
((test | star_expr)
(comp_for | (',' (test | star_expr))* [','])) )classdef: 'class' NAME ['(' [arglist] ')'] ':' suite
arglist: argument (',' argument)* [',']# The reason that keywords are test nodes instead of NAME is that using NAME# results in an ambiguity. ast.c makes sure it's a NAME.# "test '=' test" is really "keyword '=' test", but we have no such token.# These need to be in a single rule to avoid grammar that is ambiguous# to our LL(1) parser. Even though 'test' includes '*expr' in star_expr,# we explicitly match '*' here, too, to give it proper precedence.# Illegal combinations and orderings are blocked in ast.c:# multiple (test comp_for) arguments are blocked; keyword unpackings# that precede iterable unpackings are blocked; etc.argument: ( test [comp_for] |
test '=' test |
'**' test |
'*' test )comp_iter: comp_for | comp_if
comp_for: [ASYNC] 'for' exprlist 'in' or_test [comp_iter]comp_if: 'if' test_nocond [comp_iter]# not used in grammar, but may appear in "node" passed from Parser to Compilerencoding_decl: NAME
yield_expr: 'yield' [yield_arg]yield_arg: 'from' test | testlist
各种token值的定义都在Include/graminit.h（https://github.com/python/cpython/blob/master/Include/graminit.h） 。组成语法解析树的node * structs定义在Include/node.h（https://github.com/python/cpython/blob/master/Include/node.h）。
从node结构体里查询是由一组定义在 Include/token.h 的一组宏实现的：
- CHILD(node *, int)
Returns the nth child of the node using zero-offset indexing
- RCHILD(node *, int)
Returns the nth child of the node from the right side; use negative numbers!
- NCH(node *)
Number of children the node has
- STR(node *)
String representation of the node; e.g., will return : for a COLON token
- TYPE(node *)
The type of node as specified in Include/graminit.h
- REQ(node *, TYPE)
Assert that the node is the type that is expected
- LINENO(node *)
retrieve the line number of the source code that led to the creation of the parse rule; defined in Python/ast.c
我们可以通过while语法的定义了解到上面所有的示例：
while_stmt: 'while' test ':' suite ['else' ':' suite]
这段定义表示这个节点将会有如下等式成立：
TYPE(node) == while_stmt；
子节点的编号将会是4或者7，取决于'while'后面是否有一个'else'声明。通过下面的代码我们可以确定后面的第一个 ':' 是否存在以及代表什么。
(REQ(CHILD(node, 2), COLON)
抽象语法树**** (AST)
抽象语法树（AST）是程序结构的一种高抽象层次的表达，有了它我们并再不需要源代码的存在了。它可以认为是源代码等价的一种抽象表达。CPython采用Zephyr抽象语法定义语言（ASDL）（http://www.cs.princeton.edu/research/techreps/TR-554-97）来描述AST。
Python的AST定义可以在源代码的Parser/Python.asdl（https://github.com/python/cpython/blob/master/Parser/Python.asdl）找到:
-- ASDL's 7 builtin types are:-- identifier, int, string, bytes, object, singleton, constant---- singleton: None, True or False-- constant can be None, whereas None means "no value" for object.module Python{
mod = Module(stmt* body, string? docstring)
| Interactive(stmt* body)
| Expression(expr body)
-- not really an actual node but useful in Jython's typesystem.
| Suite(stmt* body)
stmt = FunctionDef(identifier name, arguments args,
stmt* body, expr* decorator_list, expr? returns,
string? docstring)
| AsyncFunctionDef(identifier name, arguments args,
stmt* body, expr* decorator_list, expr? returns,
string? docstring)
| ClassDef(identifier name,
expr* bases,
keyword* keywords,
stmt* body,
expr* decorator_list,
string? docstring)
| Return(expr? value)
| Delete(expr* targets)
| Assign(expr* targets, expr value)
| AugAssign(expr target, operator op, expr value)
-- 'simple' indicates that we annotate simple name without parens
| AnnAssign(expr target, expr annotation, expr? value, int simple)
-- use 'orelse' because else is a keyword in target languages
| For(expr target, expr iter, stmt* body, stmt* orelse)
| AsyncFor(expr target, expr iter, stmt* body, stmt* orelse)
| While(expr test, stmt* body, stmt* orelse)
| If(expr test, stmt* body, stmt* orelse)
| With(withitem* items, stmt* body)
| AsyncWith(withitem* items, stmt* body)
| Raise(expr? exc, expr? cause)
| Try(stmt* body, excepthandler* handlers, stmt* orelse, stmt* finalbody)
| Assert(expr test, expr? msg)
| Import(alias* names)
| ImportFrom(identifier? module, alias* names, int? level)
| Global(identifier* names)
| Nonlocal(identifier* names)
| Expr(expr value)
| Pass | Break | Continue
-- XXX Jython will be different
-- col_offset is the byte offset in the utf8 string the parser uses
attributes (int lineno, int col_offset)
-- BoolOp() can use left & right?
expr = BoolOp(boolop op, expr* values)
| BinOp(expr left, operator op, expr right)
| UnaryOp(unaryop op, expr operand)
| Lambda(arguments args, expr body)
| IfExp(expr test, expr body, expr orelse)
| Dict(expr* keys, expr* values)
| Set(expr* elts)
| ListComp(expr elt, comprehension* generators)
| SetComp(expr elt, comprehension* generators)
| DictComp(expr key, expr value, comprehension* generators)
| GeneratorExp(expr elt, comprehension* generators)
-- the grammar constrains where yield expressions can occur
| Await(expr value)
| Yield(expr? value)
| YieldFrom(expr value)
-- need sequences for compare to distinguish between
-- x < 4 < 3 and (x < 4) < 3
| Compare(expr left, cmpop* ops, expr* comparators)
| Call(expr func, expr* args, keyword* keywords)
| Num(object n) -- a number as a PyObject.
| Str(string s) -- need to specify raw, unicode, etc?
| FormattedValue(expr value, int? conversion, expr? format_spec)
| JoinedStr(expr* values)
| Bytes(bytes s)
| NameConstant(singleton value)
| Ellipsis
| Constant(constant value)
-- the following expression can appear in assignment context
| Attribute(expr value, identifier attr, expr_context ctx)
| Subscript(expr value, slice slice, expr_context ctx)
| Starred(expr value, expr_context ctx)
| Name(identifier id, expr_context ctx)
| List(expr* elts, expr_context ctx)
| Tuple(expr* elts, expr_context ctx)
-- col_offset is the byte offset in the utf8 string the parser uses
attributes (int lineno, int col_offset)
expr_context = Load | Store | Del | AugLoad | AugStore | Param
slice = Slice(expr? lower, expr? upper, expr? step)
| ExtSlice(slice* dims)
| Index(expr value)
boolop = And | Or
operator = Add | Sub | Mult | MatMult | Div | Mod | Pow | LShift
| RShift | BitOr | BitXor | BitAnd | FloorDiv
unaryop = Invert | Not | UAdd | USub
cmpop = Eq | NotEq | Lt | LtE | Gt | GtE | Is | IsNot | In | NotIn
comprehension = (expr target, expr iter, expr* ifs, int is_async)
excepthandler = ExceptHandler(expr? type, identifier? name, stmt* body)
attributes (int lineno, int col_offset)
arguments = (arg* args, arg? vararg, arg* kwonlyargs, expr* kw_defaults,
arg? kwarg, expr* defaults)
arg = (identifier arg, expr? annotation)
attributes (int lineno, int col_offset)
-- keyword arguments supplied to call (NULL identifier for **kwargs)
keyword = (identifier? arg, expr value)
-- import name with optional 'as' alias.
alias = (identifier name, identifier? asname)
withitem = (expr context_expr, expr? optional_vars)}
每种AST节点（声明，表达式，一些特殊类型例如：列表推导式、异常处理handler）都被ASDL描述定义。大多数的AST定义都对应一种特定的源码结构，例如一个'if'定义或者一个属性查找。这些定义都是和具体实现Python的语言无关的，也就是说无论CPython、PyPy、Jyphon甚至IronPython，都是用的同样的ASDL。
下面的Python ASDL结构展示了Python的函数定义相关语法结构和实现：
module Python{
stmt = FunctionDef(identifier name, arguments args, stmt* body,
expr* decorators)
| Return(expr? value) | Yield(expr value)
attributes (int lineno)}
上面的示例描述了3种不同的语法结构：def函数定义、return声明、yield声明。这三种语法结构的语法声明被用 '|' 分割。它们接受的参数类型和个数各不相同。
每个参数定义前面还带了不同的修饰符（这点和正则很像）来说明需要多少个参数：
'?'说明这个参数是可有可无的（0~1个）；
'*'说明这个参数的数量是0或者更多（≥ 0）；
没有修饰符表示这个参数是必须且只能有一个的。
例如，函数定义（def）接受一个 identifier 作为函数名，'arguments'作为参数，0或者多个stmt作为函数体，0或者多个expr作为装饰器。
请注意这里的'arguments'，不像大家可能会认为的和stmt一样，它是作为一个node类型会由一个单独的AST定义所表示。
所有的这三种语法结构类型都有一个 'attributes' 参数，所以 'attributes' 前面没有 '|' 。
上面的ASDL会被翻译生成如下的C语言结构体类型：
typedef struct _stmt *stmt_ty;struct _stmt {
enum { FunctionDef_kind=1, Return_kind=2, Yield_kind=3 } kind;
union {
struct {
identifier name;
arguments_ty args;
asdl_seq *body;
} FunctionDef;
struct {
expr_ty value;
} Return;
struct {
expr_ty value;
} Yield;
} v;
int lineno;
}
一同被生成的还有一些构造函数，它们会给这些语法结构分配一个stmt_ty结构体，并附带一些必要的初始化。'kind' enum字段表明下面声明的是哪种union。FunctionDef() 的构造函数将会吧'kind'设置成 FunctionDef_kind 并且初始化 'name', 'args', 'body', 和 'attributes' 字段。
未完待续
