Node classes¶

classast.AST¶

This is the base of all AST node classes. The actual node classes arederived from theParser/Python.asdl file, which is reproducedbelow. They are defined in the_ast Cmodule and re-exported inast.

There is one class defined for each left-hand side symbol in the abstractgrammar (for example,ast.stmt orast.expr). In addition,there is one class defined for each constructor on the right-hand side; theseclasses inherit from the classes for the left-hand side trees. For example,ast.BinOp inherits fromast.expr. For production ruleswith alternatives (aka “sums”), the left-hand side class is abstract: onlyinstances of specific constructor nodes are ever created.

_fields¶

Each concrete class has an attribute_fields which gives the namesof all child nodes.

Each instance of a concrete class has one attribute for each child node,of the type as defined in the grammar. For example,ast.BinOpinstances have an attributeleft of typeast.expr.

If these attributes are marked as optional in the grammar (using aquestion mark), the value might beNone. If the attributes can havezero-or-more values (marked with an asterisk), the values are representedas Python lists. All possible attributes must be present and have validvalues when compiling an AST withcompile().

lineno¶

col_offset¶

end_lineno¶

end_col_offset¶

Instances ofast.expr andast.stmt subclasses havelineno,col_offset,lineno, andcol_offsetattributes. Thelineno andend_lineno are the first andlast line numbers of source text span (1-indexed so the first line is line 1)and thecol_offset andend_col_offset are the correspondingUTF-8 byte offsets of the first and last tokens that generated the node.The UTF-8 offset is recorded because the parser uses UTF-8 internally.

Note that the end positions are not required by the compiler and aretherefore optional. The end offset isafter the last symbol, for exampleone can get the source segment of a one-line expression node usingsource_line[node.col_offset:node.end_col_offset].

The constructor of a classast.T parses its arguments as follows:

• If there are positional arguments, there must be as many as there are itemsinT._fields; they will be assigned as attributes of these names.

• If there are keyword arguments, they will set the attributes of the samenames to the given values.

For example, to create and populate anast.UnaryOp node, you coulduse

node=ast.UnaryOp()node.op=ast.USub()node.operand=ast.Constant()node.operand.value=5node.operand.lineno=0node.operand.col_offset=0node.lineno=0node.col_offset=0

or the more compact

node=ast.UnaryOp(ast.USub(),ast.Constant(5,lineno=0,col_offset=0),lineno=0,col_offset=0)

Abstract Grammar¶

The abstract grammar is currently defined as follows:

-- ASDL's 5 builtin types are:-- identifier, int, string, object, constantmodule Python{    mod = Module(stmt* body, type_ignore *type_ignores)        | Interactive(stmt* body)        | Expression(expr body)        | FunctionType(expr* argtypes, expr returns)        -- 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? type_comment)          | AsyncFunctionDef(identifier name, arguments args,                             stmt* body, expr* decorator_list, expr? returns,                             string? type_comment)          | ClassDef(identifier name,             expr* bases,             keyword* keywords,             stmt* body,             expr* decorator_list)          | Return(expr? value)          | Delete(expr* targets)          | Assign(expr* targets, expr value, string? type_comment)          | 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, string? type_comment)          | AsyncFor(expr target, expr iter, stmt* body, stmt* orelse, string? type_comment)          | While(expr test, stmt* body, stmt* orelse)          | If(expr test, stmt* body, stmt* orelse)          | With(withitem* items, stmt* body, string? type_comment)          | AsyncWith(withitem* items, stmt* body, string? type_comment)          | 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, int? end_lineno, int? end_col_offset)          -- BoolOp() can use left & right?    expr = BoolOp(boolop op, expr* values)         | NamedExpr(expr target, expr value)         | 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)         | FormattedValue(expr value, int? conversion, expr? format_spec)         | JoinedStr(expr* values)         | Constant(constant value, string? kind)         -- 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, int? end_lineno, int? end_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, int? end_lineno, int? end_col_offset)    arguments = (arg* posonlyargs, arg* args, arg? vararg, arg* kwonlyargs,                 expr* kw_defaults, arg? kwarg, expr* defaults)    arg = (identifier arg, expr? annotation, string? type_comment)           attributes (int lineno, int col_offset, int? end_lineno, int? end_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)    type_ignore = TypeIgnore(int lineno, string tag)}

ast Helpers¶

Apart from the node classes, theast module defines these utility functionsand classes for traversing abstract syntax trees:

ast.parse(source,filename='<unknown>',mode='exec',*,type_comments=False,feature_version=None)¶

Parse the source into an AST node. Equivalent tocompile(source,filename,mode,ast.PyCF_ONLY_AST).

Iftype_comments=True is given, the parser is modified to checkand return type comments as specified byPEP 484 andPEP 526.This is equivalent to addingast.PyCF_TYPE_COMMENTS to theflags passed tocompile(). This will report syntax errorsfor misplaced type comments. Without this flag, type comments willbe ignored, and thetype_comment field on selected AST nodeswill always beNone. In addition, the locations of#type:ignore comments will be returned as thetype_ignoresattribute ofModule (otherwise it is always an empty list).

In addition, ifmode is'func_type', the input syntax ismodified to correspond toPEP 484 “signature type comments”,e.g.(str,int)->List[str].

Also, settingfeature_version to a tuple(major,minor)will attempt to parse using that Python version’s grammar.Currentlymajor must equal to3. For example, settingfeature_version=(3,4) will allow the use ofasync andawait as variable names. The lowest supported version is(3,4); the highest issys.version_info[0:2].

Warning

It is possible to crash the Python interpreter with asufficiently large/complex string due to stack depth limitationsin Python’s AST compiler.

Changed in version 3.8:Addedtype_comments,mode='func_type' andfeature_version.

ast.literal_eval(node_or_string)¶

Safely evaluate an expression node or a string containing a Python literal orcontainer display. The string or node provided may only consist of thefollowing Python literal structures: strings, bytes, numbers, tuples, lists,dicts, sets, booleans, andNone.

This can be used for safely evaluating strings containing Python values fromuntrusted sources without the need to parse the values oneself. It is notcapable of evaluating arbitrarily complex expressions, for example involvingoperators or indexing.

Warning

It is possible to crash the Python interpreter with asufficiently large/complex string due to stack depth limitationsin Python’s AST compiler.

Changed in version 3.2:Now allows bytes and set literals.

ast.get_docstring(node,clean=True)¶

Return the docstring of the givennode (which must be aFunctionDef,AsyncFunctionDef,ClassDef,orModule node), orNone if it has no docstring.Ifclean is true, clean up the docstring’s indentation withinspect.cleandoc().

Changed in version 3.5:AsyncFunctionDef is now supported.

ast.get_source_segment(source,node,*,padded=False)¶

Get source code segment of thesource that generatednode.If some location information (lineno,end_lineno,col_offset, orend_col_offset) is missing, returnNone.

Ifpadded isTrue, the first line of a multi-line statement willbe padded with spaces to match its original position.

New in version 3.8.

ast.fix_missing_locations(node)¶

When you compile a node tree withcompile(), the compiler expectslineno andcol_offset attributes for every node that supportsthem. This is rather tedious to fill in for generated nodes, so this helperadds these attributes recursively where not already set, by setting them tothe values of the parent node. It works recursively starting atnode.

ast.increment_lineno(node,n=1)¶

Increment the line number and end line number of each node in the treestarting atnode byn. This is useful to “move code” to a differentlocation in a file.

ast.copy_location(new_node,old_node)¶

Copy source location (lineno,col_offset,end_lineno,andend_col_offset) fromold_node tonew_node if possible,and returnnew_node.

ast.iter_fields(node)¶

Yield a tuple of(fieldname,value) for each field innode._fieldsthat is present onnode.

ast.iter_child_nodes(node)¶

Yield all direct child nodes ofnode, that is, all fields that are nodesand all items of fields that are lists of nodes.

ast.walk(node)¶

Recursively yield all descendant nodes in the tree starting atnode(includingnode itself), in no specified order. This is useful if you onlywant to modify nodes in place and don’t care about the context.

classast.NodeVisitor¶

A node visitor base class that walks the abstract syntax tree and calls avisitor function for every node found. This function may return a valuewhich is forwarded by thevisit() method.

This class is meant to be subclassed, with the subclass adding visitormethods.

visit(node)¶

Visit a node. The default implementation calls the method calledself.visit_classname whereclassname is the name of the nodeclass, orgeneric_visit() if that method doesn’t exist.

generic_visit(node)¶

This visitor callsvisit() on all children of the node.

Note that child nodes of nodes that have a custom visitor method won’t bevisited unless the visitor callsgeneric_visit() or visits themitself.

Don’t use theNodeVisitor if you want to apply changes to nodesduring traversal. For this a special visitor exists(NodeTransformer) that allows modifications.

Deprecated since version 3.8:Methodsvisit_Num(),visit_Str(),visit_Bytes(),visit_NameConstant() andvisit_Ellipsis() are deprecatednow and will not be called in future Python versions. Add thevisit_Constant() method to handle all constant nodes.

classast.NodeTransformer¶

ANodeVisitor subclass that walks the abstract syntax tree andallows modification of nodes.

TheNodeTransformer will walk the AST and use the return value ofthe visitor methods to replace or remove the old node. If the return valueof the visitor method isNone, the node will be removed from itslocation, otherwise it is replaced with the return value. The return valuemay be the original node in which case no replacement takes place.

Here is an example transformer that rewrites all occurrences of name lookups(foo) todata['foo']:

classRewriteName(NodeTransformer):defvisit_Name(self,node):returnSubscript(value=Name(id='data',ctx=Load()),slice=Index(value=Constant(value=node.id)),ctx=node.ctx)

Keep in mind that if the node you’re operating on has child nodes you musteither transform the child nodes yourself or call thegeneric_visit()method for the node first.

For nodes that were part of a collection of statements (that applies to allstatement nodes), the visitor may also return a list of nodes rather thanjust a single node.

IfNodeTransformer introduces new nodes (that weren’t part oforiginal tree) without giving them location information (such aslineno),fix_missing_locations() should be called withthe new sub-tree to recalculate the location information:

tree=ast.parse('foo',mode='eval')new_tree=fix_missing_locations(RewriteName().visit(tree))

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 innode. This is mainly useful fordebugging purposes. Ifannotate_fields is true (by default),the returned string will show the names and the values for fields.Ifannotate_fields is false, the result string will be more compact byomitting unambiguous field names. Attributes such as linenumbers and column offsets are not dumped by default. If this is wanted,include_attributes can be set to true.