### 導航 - [索引](../genindex.xhtml "總目錄") - [模塊](../py-modindex.xhtml "Python 模塊索引") | - [下一頁](symtable.xhtml "symtable --- Access to the compiler's symbol tables") | - [上一頁](parser.xhtml "parser --- Access Python parse trees") | -  - [Python](https://www.python.org/) ? - zh\_CN 3.7.3 [文檔](../index.xhtml) ? - [Python 標準庫](index.xhtml) ? - [Python 語言服務](language.xhtml) ? - $('.inline-search').show(0); | # [`ast`](#module-ast "ast: Abstract Syntax Tree classes and manipulation.") --- 抽象語法樹 **源代碼：** [Lib/ast.py](https://github.com/python/cpython/tree/3.7/Lib/ast.py) \[https://github.com/python/cpython/tree/3.7/Lib/ast.py\] - - - - - - [`ast`](#module-ast "ast: Abstract Syntax Tree classes and manipulation.") 模塊幫助 Python 程序處理 Python 語法的抽象語法樹。抽象語法或許會隨著 Python 的更新發布而改變；該模塊能夠幫助理解當前語法在編程層面的樣貌。 抽象語法樹可通過將 `ast.PyCF_ONLY_AST` 作為旗標傳遞給 [`compile()`](functions.xhtml#compile "compile") 內置函數來生成，或是使用此模塊中提供的 [`parse()`](#ast.parse "ast.parse") 輔助函數。返回結果將是一個對象樹，，其中的類都繼承自 [`ast.AST`](#ast.AST "ast.AST")。抽象語法樹可被內置的 [`compile()`](functions.xhtml#compile "compile") 函數編譯為一個 Python 代碼對象。 ## 節點類 *class* `ast.``AST`這是所有 AST 節點類的基類。實際上，這些節點類派生自 `Parser/Python.asdl` 文件，其中定義的語法樹示例 [如下](#abstract-grammar)。它們在 C 語言模塊 `_ast` 中定義，并被導出至 [`ast`](#module-ast "ast: Abstract Syntax Tree classes and manipulation.") 模塊。 抽象語法定義的每個左側符號(比方說， `ast.stmt` 或者 `ast.expr`)定義了一個類。另外，在抽象語法定義的右側，對每一個構造器也定義了一個類；這些類繼承自樹左側的類。比如，`ast.BinOp` 繼承自 `ast.expr`。對于多分支產生式(也就是"和規則")，樹右側的類是抽象的；只有特定構造器結點的實例能被構造。 `_fields`每個具體類都有個屬性 [`_fields`](#ast.AST._fields "ast.AST._fields"), 用來給出所有子節點的名字。 每個具體類的實例對它每個子節點都有一個屬性，對應類型如文法中所定義。比如，`ast.BinOp` 的實例有個屬性 `left`，類型是 `ast.expr`. 如果這些屬性在文法中標記為可選（使用問號），對應值可能會是 `None`。如果這些屬性有零或多個（用星號標記），對應值會用Python的列表來表示。所有可能的屬性必須在用 [`compile()`](functions.xhtml#compile "compile") 編譯得到AST時給出，且是有效的值。 `lineno``col_offset``ast.expr` 和 `ast.stmt` 子類的實例有 [`lineno`](#ast.AST.lineno "ast.AST.lineno") 和 [`col_offset`](#ast.AST.col_offset "ast.AST.col_offset") 屬性。[`lineno`](#ast.AST.lineno "ast.AST.lineno") 是源代碼的行數(從1開始, 所以第一行行數是1)，而 [`col_offset`](#ast.AST.col_offset "ast.AST.col_offset") 是該生成節點第一個token的UTF-8字節偏移量。記錄下UTF-8偏移量的原因是parser內部使用UTF-8。 一個類的構造器 `ast.T` 像下面這樣parse它的參數。 - 如果有位置參數，它們必須和 `T._fields` 中的元素一樣多；他們會像這些名字的屬性一樣被賦值。 - 如果有關鍵字參數，它們必須被設為和給定值同名的屬性。 比方說，要創建和填充節點 `ast.UnaryOp`，你得用 ``` node = ast.UnaryOp() node.op = ast.USub() node.operand = ast.Num() node.operand.n = 5 node.operand.lineno = 0 node.operand.col_offset = 0 node.lineno = 0 node.col_offset = 0 ``` 或者更緊湊點 ``` node = ast.UnaryOp(ast.USub(), ast.Num(5, lineno=0, col_offset=0), lineno=0, col_offset=0) ``` ## 抽象文法 抽象文法目前定義如下 ``` -- 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) | 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) | AsyncFunctionDef(identifier name, arguments args, stmt* body, expr* decorator_list, expr? returns) | ClassDef(identifier name, expr* bases, keyword* keywords, stmt* body, expr* decorator_list) | 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) } ``` ## [`ast`](#module-ast "ast: Abstract Syntax Tree classes and manipulation.") 中的輔助函數 除了節點類， [`ast`](#module-ast "ast: Abstract Syntax Tree classes and manipulation.") 模塊里為遍歷抽象語法樹定義了這些工具函數和類: `ast.``parse`(*source*, *filename='<unknown>'*, *mode='exec'*)把源碼解析為AST節點。和 `compile(source, filename, mode,ast.PyCF_ONLY_AST)` 等價。 警告 足夠復雜或是巨大的字符串可能導致Python解釋器的崩潰，因為Python的AST編譯器是有棧深限制的。 `ast.``literal_eval`(*node\_or\_string*)Safely evaluate an expression node or a string containing a Python literal or container display. The string or node provided may only consist of the following Python literal structures: strings, bytes, numbers, tuples, lists, dicts, sets, booleans, and `None`. This can be used for safely evaluating strings containing Python values from untrusted sources without the need to parse the values oneself. It is not capable of evaluating arbitrarily complex expressions, for example involving operators or indexing. 警告 足夠復雜或是巨大的字符串可能導致Python解釋器的崩潰，因為Python的AST編譯器是有棧深限制的。 在 3.2 版更改: Now allows bytes and set literals. `ast.``get_docstring`(*node*, *clean=True*)Return the docstring of the given *node* (which must be a `FunctionDef`, `AsyncFunctionDef`, `ClassDef`, or `Module` node), or `None` if it has no docstring. If *clean* is true, clean up the docstring's indentation with [`inspect.cleandoc()`](inspect.xhtml#inspect.cleandoc "inspect.cleandoc"). 在 3.5 版更改: `AsyncFunctionDef` is now supported. `ast.``fix_missing_locations`(*node*)When you compile a node tree with [`compile()`](functions.xhtml#compile "compile"), the compiler expects `lineno` and `col_offset` attributes for every node that supports them. This is rather tedious to fill in for generated nodes, so this helper adds these attributes recursively where not already set, by setting them to the values of the parent node. It works recursively starting at *node*. `ast.``increment_lineno`(*node*, *n=1*)Increment the line number of each node in the tree starting at *node* by *n*. This is useful to "move code" to a different location in a file. `ast.``copy_location`(*new\_node*, *old\_node*)Copy source location (`lineno` and `col_offset`) from *old\_node*to *new\_node* if possible, and return *new\_node*. `ast.``iter_fields`(*node*)Yield a tuple of `(fieldname, value)` for each field in `node._fields`that is present on *node*. `ast.``iter_child_nodes`(*node*)Yield all direct child nodes of *node*, that is, all fields that are nodes and all items of fields that are lists of nodes. `ast.``walk`(*node*)Recursively yield all descendant nodes in the tree starting at *node*(including *node* itself), in no specified order. This is useful if you only want to modify nodes in place and don't care about the context. *class* `ast.``NodeVisitor`A node visitor base class that walks the abstract syntax tree and calls a visitor function for every node found. This function may return a value which is forwarded by the [`visit()`](#ast.NodeVisitor.visit "ast.NodeVisitor.visit") method. This class is meant to be subclassed, with the subclass adding visitor methods. `visit`(*node*)Visit a node. The default implementation calls the method called `self.visit_classname` where *classname* is the name of the node class, or [`generic_visit()`](#ast.NodeVisitor.generic_visit "ast.NodeVisitor.generic_visit") if that method doesn't exist. `generic_visit`(*node*)This visitor calls [`visit()`](#ast.NodeVisitor.visit "ast.NodeVisitor.visit") on all children of the node. Note that child nodes of nodes that have a custom visitor method won't be visited unless the visitor calls [`generic_visit()`](#ast.NodeVisitor.generic_visit "ast.NodeVisitor.generic_visit") or visits them itself. Don't use the [`NodeVisitor`](#ast.NodeVisitor "ast.NodeVisitor") if you want to apply changes to nodes during traversal. For this a special visitor exists ([`NodeTransformer`](#ast.NodeTransformer "ast.NodeTransformer")) that allows modifications. *class* `ast.``NodeTransformer`A [`NodeVisitor`](#ast.NodeVisitor "ast.NodeVisitor") subclass that walks the abstract syntax tree and allows modification of nodes. The [`NodeTransformer`](#ast.NodeTransformer "ast.NodeTransformer") will walk the AST and use the return value of the visitor methods to replace or remove the old node. If the return value of the visitor method is `None`, the node will be removed from its location, otherwise it is replaced with the return value. The return value may be the original node in which case no replacement takes place. Here is an example transformer that rewrites all occurrences of name lookups (`foo`) to `data['foo']`: ``` class RewriteName(NodeTransformer): def visit_Name(self, node): return copy_location(Subscript( value=Name(id='data', ctx=Load()), slice=Index(value=Str(s=node.id)), ctx=node.ctx ), node) ``` Keep in mind that if the node you're operating on has child nodes you must either transform the child nodes yourself or call the `generic_visit()`method for the node first. For nodes that were part of a collection of statements (that applies to all statement nodes), the visitor may also return a list of nodes rather than just a single node. Usually you use the transformer like this: ``` node = YourTransformer().visit(node) ``` `ast.``dump`(*node*, *annotate\_fields=True*, *include\_attributes=False*)Return a formatted dump of the tree in *node*. This is mainly useful for debugging purposes. The returned string will show the names and the values for fields. This makes the code impossible to evaluate, so if evaluation is wanted *annotate\_fields* must be set to `False`. Attributes such as line numbers and column offsets are not dumped by default. If this is wanted, *include\_attributes* can be set to `True`. 參見 [Green Tree Snakes](https://greentreesnakes.readthedocs.io/) \[https://greentreesnakes.readthedocs.io/\], an external documentation resource, has good details on working with Python ASTs. ### 導航 - [索引](../genindex.xhtml "總目錄") - [模塊](../py-modindex.xhtml "Python 模塊索引") | - [下一頁](symtable.xhtml "symtable --- Access to the compiler's symbol tables") | - [上一頁](parser.xhtml "parser --- Access Python parse trees") | -  - [Python](https://www.python.org/) ? - zh\_CN 3.7.3 [文檔](../index.xhtml) ? - [Python 標準庫](index.xhtml) ? - [Python 語言服務](language.xhtml) ? - $('.inline-search').show(0); | ? [版權所有](../copyright.xhtml) 2001-2019, Python Software Foundation. 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