cpython/Lib/ast.py

"""
    ast
    ~~~

    The `ast` module helps Python applications to process trees of the Python
    abstract syntax grammar.  The abstract syntax itself might change with
    each Python release; this module helps to find out programmatically what
    the current grammar looks like and allows modifications of it.

    An abstract syntax tree can be generated by passing `ast.PyCF_ONLY_AST` as
    a flag to the `compile()` builtin function or by using the `parse()`
    function from this module.  The result will be a tree of objects whose
    classes all inherit from `ast.AST`.

    A modified abstract syntax tree can be compiled into a Python code object
    using the built-in `compile()` function.

    Additionally various helper functions are provided that make working with
    the trees simpler.  The main intention of the helper functions and this
    module in general is to provide an easy to use interface for libraries
    that work tightly with the python syntax (template engines for example).


    :copyright: Copyright 2008 by Armin Ronacher.
    :license: Python License.
"""
import sys
from _ast import *


def parse(source, filename='<unknown>', mode='exec', *,
          type_comments=False, feature_version=None):
    """
    Parse the source into an AST node.
    Equivalent to compile(source, filename, mode, PyCF_ONLY_AST).
    Pass type_comments=True to get back type comments where the syntax allows.
    """
    flags = PyCF_ONLY_AST
    if type_comments:
        flags |= PyCF_TYPE_COMMENTS
    if isinstance(feature_version, tuple):
        major, minor = feature_version  # Should be a 2-tuple.
        assert major == 3
        feature_version = minor
    elif feature_version is None:
        feature_version = -1
    # Else it should be an int giving the minor version for 3.x.
    return compile(source, filename, mode, flags,
                   _feature_version=feature_version)


def literal_eval(node_or_string):
    """
    Safely evaluate an expression node or a string containing a Python
    expression.  The string or node provided may only consist of the following
    Python literal structures: strings, bytes, numbers, tuples, lists, dicts,
    sets, booleans, and None.
    """
    if isinstance(node_or_string, str):
        node_or_string = parse(node_or_string, mode='eval')
    if isinstance(node_or_string, Expression):
        node_or_string = node_or_string.body
    def _convert_num(node):
        if isinstance(node, Constant):
            if type(node.value) in (int, float, complex):
                return node.value
        raise ValueError('malformed node or string: ' + repr(node))
    def _convert_signed_num(node):
        if isinstance(node, UnaryOp) and isinstance(node.op, (UAdd, USub)):
            operand = _convert_num(node.operand)
            if isinstance(node.op, UAdd):
                return + operand
            else:
                return - operand
        return _convert_num(node)
    def _convert(node):
        if isinstance(node, Constant):
            return node.value
        elif isinstance(node, Tuple):
            return tuple(map(_convert, node.elts))
        elif isinstance(node, List):
            return list(map(_convert, node.elts))
        elif isinstance(node, Set):
            return set(map(_convert, node.elts))
        elif isinstance(node, Dict):
            return dict(zip(map(_convert, node.keys),
                            map(_convert, node.values)))
        elif isinstance(node, BinOp) and isinstance(node.op, (Add, Sub)):
            left = _convert_signed_num(node.left)
            right = _convert_num(node.right)
            if isinstance(left, (int, float)) and isinstance(right, complex):
                if isinstance(node.op, Add):
                    return left + right
                else:
                    return left - right
        return _convert_signed_num(node)
    return _convert(node_or_string)


def dump(node, annotate_fields=True, include_attributes=False, *, indent=None):
    """
    Return a formatted dump of the tree in node.  This is mainly useful for
    debugging purposes.  If annotate_fields is true (by default),
    the returned string will show the names and the values for fields.
    If annotate_fields is false, the result string will be more compact by
    omitting unambiguous field names.  Attributes such as line
    numbers and column offsets are not dumped by default.  If this is wanted,
    include_attributes can be set to true.  If indent is a non-negative
    integer or string, then the tree will be pretty-printed with that indent
    level. None (the default) selects the single line representation.
    """
    def _format(node, level=0):
        if indent is not None:
            level += 1
            prefix = '\n' + indent * level
            sep = ',\n' + indent * level
        else:
            prefix = ''
            sep = ', '
        if isinstance(node, AST):
            args = []
            allsimple = True
            keywords = annotate_fields
            for field in node._fields:
                try:
                    value = getattr(node, field)
                except AttributeError:
                    keywords = True
                else:
                    value, simple = _format(value, level)
                    allsimple = allsimple and simple
                    if keywords:
                        args.append('%s=%s' % (field, value))
                    else:
                        args.append(value)
            if include_attributes and node._attributes:
                for attr in node._attributes:
                    try:
                        value = getattr(node, attr)
                    except AttributeError:
                        pass
                    else:
                        value, simple = _format(value, level)
                        allsimple = allsimple and simple
                        args.append('%s=%s' % (attr, value))
            if allsimple and len(args) <= 3:
                return '%s(%s)' % (node.__class__.__name__, ', '.join(args)), not args
            return '%s(%s%s)' % (node.__class__.__name__, prefix, sep.join(args)), False
        elif isinstance(node, list):
            if not node:
                return '[]', True
            return '[%s%s]' % (prefix, sep.join(_format(x, level)[0] for x in node)), False
        return repr(node), True

    if not isinstance(node, AST):
        raise TypeError('expected AST, got %r' % node.__class__.__name__)
    if indent is not None and not isinstance(indent, str):
        indent = ' ' * indent
    return _format(node)[0]


def copy_location(new_node, old_node):
    """
    Copy source location (`lineno`, `col_offset`, `end_lineno`, and `end_col_offset`
    attributes) from *old_node* to *new_node* if possible, and return *new_node*.
    """
    for attr in 'lineno', 'col_offset', 'end_lineno', 'end_col_offset':
        if attr in old_node._attributes and attr in new_node._attributes \
           and hasattr(old_node, attr):
            setattr(new_node, attr, getattr(old_node, attr))
    return new_node


def fix_missing_locations(node):
    """
    When you compile a node tree with 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*.
    """
    def _fix(node, lineno, col_offset, end_lineno, end_col_offset):
        if 'lineno' in node._attributes:
            if not hasattr(node, 'lineno'):
                node.lineno = lineno
            else:
                lineno = node.lineno
        if 'end_lineno' in node._attributes:
            if not hasattr(node, 'end_lineno'):
                node.end_lineno = end_lineno
            else:
                end_lineno = node.end_lineno
        if 'col_offset' in node._attributes:
            if not hasattr(node, 'col_offset'):
                node.col_offset = col_offset
            else:
                col_offset = node.col_offset
        if 'end_col_offset' in node._attributes:
            if not hasattr(node, 'end_col_offset'):
                node.end_col_offset = end_col_offset
            else:
                end_col_offset = node.end_col_offset
        for child in iter_child_nodes(node):
            _fix(child, lineno, col_offset, end_lineno, end_col_offset)
    _fix(node, 1, 0, 1, 0)
    return node


def increment_lineno(node, n=1):
    """
    Increment the line number and end 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.
    """
    for child in walk(node):
        if 'lineno' in child._attributes:
            child.lineno = getattr(child, 'lineno', 0) + n
        if 'end_lineno' in child._attributes:
            child.end_lineno = getattr(child, 'end_lineno', 0) + n
    return node


def iter_fields(node):
    """
    Yield a tuple of ``(fieldname, value)`` for each field in ``node._fields``
    that is present on *node*.
    """
    for field in node._fields:
        try:
            yield field, getattr(node, field)
        except AttributeError:
            pass


def 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.
    """
    for name, field in iter_fields(node):
        if isinstance(field, AST):
            yield field
        elif isinstance(field, list):
            for item in field:
                if isinstance(item, AST):
                    yield item


def get_docstring(node, clean=True):
    """
    Return the docstring for the given node or None if no docstring can
    be found.  If the node provided does not have docstrings a TypeError
    will be raised.

    If *clean* is `True`, all tabs are expanded to spaces and any whitespace
    that can be uniformly removed from the second line onwards is removed.
    """
    if not isinstance(node, (AsyncFunctionDef, FunctionDef, ClassDef, Module)):
        raise TypeError("%r can't have docstrings" % node.__class__.__name__)
    if not(node.body and isinstance(node.body[0], Expr)):
        return None
    node = node.body[0].value
    if isinstance(node, Str):
        text = node.s
    elif isinstance(node, Constant) and isinstance(node.value, str):
        text = node.value
    else:
        return None
    if clean:
        import inspect
        text = inspect.cleandoc(text)
    return text


def _splitlines_no_ff(source):
    """Split a string into lines ignoring form feed and other chars.

    This mimics how the Python parser splits source code.
    """
    idx = 0
    lines = []
    next_line = ''
    while idx < len(source):
        c = source[idx]
        next_line += c
        idx += 1
        # Keep \r\n together
        if c == '\r' and idx < len(source) and source[idx] == '\n':
            next_line += '\n'
            idx += 1
        if c in '\r\n':
            lines.append(next_line)
            next_line = ''

    if next_line:
        lines.append(next_line)
    return lines


def _pad_whitespace(source):
    """Replace all chars except '\f\t' in a line with spaces."""
    result = ''
    for c in source:
        if c in '\f\t':
            result += c
        else:
            result += ' '
    return result


def get_source_segment(source, node, *, padded=False):
    """Get source code segment of the *source* that generated *node*.

    If some location information (`lineno`, `end_lineno`, `col_offset`,
    or `end_col_offset`) is missing, return None.

    If *padded* is `True`, the first line of a multi-line statement will
    be padded with spaces to match its original position.
    """
    try:
        lineno = node.lineno - 1
        end_lineno = node.end_lineno - 1
        col_offset = node.col_offset
        end_col_offset = node.end_col_offset
    except AttributeError:
        return None

    lines = _splitlines_no_ff(source)
    if end_lineno == lineno:
        return lines[lineno].encode()[col_offset:end_col_offset].decode()

    if padded:
        padding = _pad_whitespace(lines[lineno].encode()[:col_offset].decode())
    else:
        padding = ''

    first = padding + lines[lineno].encode()[col_offset:].decode()
    last = lines[end_lineno].encode()[:end_col_offset].decode()
    lines = lines[lineno+1:end_lineno]

    lines.insert(0, first)
    lines.append(last)
    return ''.join(lines)


def 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.
    """
    from collections import deque
    todo = deque([node])
    while todo:
        node = todo.popleft()
        todo.extend(iter_child_nodes(node))
        yield node


class NodeVisitor(object):
    """
    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` method.

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

    Per default the visitor functions for the nodes are ``'visit_'`` +
    class name of the node.  So a `TryFinally` node visit function would
    be `visit_TryFinally`.  This behavior can be changed by overriding
    the `visit` method.  If no visitor function exists for a node
    (return value `None`) the `generic_visit` visitor is used instead.

    Don't use the `NodeVisitor` if you want to apply changes to nodes during
    traversing.  For this a special visitor exists (`NodeTransformer`) that
    allows modifications.
    """

    def visit(self, node):
        """Visit a node."""
        method = 'visit_' + node.__class__.__name__
        visitor = getattr(self, method, self.generic_visit)
        return visitor(node)

    def generic_visit(self, node):
        """Called if no explicit visitor function exists for a node."""
        for field, value in iter_fields(node):
            if isinstance(value, list):
                for item in value:
                    if isinstance(item, AST):
                        self.visit(item)
            elif isinstance(value, AST):
                self.visit(value)

    def visit_Constant(self, node):
        value = node.value
        type_name = _const_node_type_names.get(type(value))
        if type_name is None:
            for cls, name in _const_node_type_names.items():
                if isinstance(value, cls):
                    type_name = name
                    break
        if type_name is not None:
            method = 'visit_' + type_name
            try:
                visitor = getattr(self, method)
            except AttributeError:
                pass
            else:
                import warnings
                warnings.warn(f"{method} is deprecated; add visit_Constant",
                              DeprecationWarning, 2)
                return visitor(node)
        return self.generic_visit(node)


class NodeTransformer(NodeVisitor):
    """
    A :class:`NodeVisitor` subclass that walks the abstract syntax tree and
    allows modification of nodes.

    The `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 :meth:`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)
    """

    def generic_visit(self, node):
        for field, old_value in iter_fields(node):
            if isinstance(old_value, list):
                new_values = []
                for value in old_value:
                    if isinstance(value, AST):
                        value = self.visit(value)
                        if value is None:
                            continue
                        elif not isinstance(value, AST):
                            new_values.extend(value)
                            continue
                    new_values.append(value)
                old_value[:] = new_values
            elif isinstance(old_value, AST):
                new_node = self.visit(old_value)
                if new_node is None:
                    delattr(node, field)
                else:
                    setattr(node, field, new_node)
        return node


# The following code is for backward compatibility.
# It will be removed in future.

def _getter(self):
    return self.value

def _setter(self, value):
    self.value = value

Constant.n = property(_getter, _setter)
Constant.s = property(_getter, _setter)

class _ABC(type):

    def __instancecheck__(cls, inst):
        if not isinstance(inst, Constant):
            return False
        if cls in _const_types:
            try:
                value = inst.value
            except AttributeError:
                return False
            else:
                return (
                    isinstance(value, _const_types[cls]) and
                    not isinstance(value, _const_types_not.get(cls, ()))
                )
        return type.__instancecheck__(cls, inst)

def _new(cls, *args, **kwargs):
    if cls in _const_types:
        return Constant(*args, **kwargs)
    return Constant.__new__(cls, *args, **kwargs)

class Num(Constant, metaclass=_ABC):
    _fields = ('n',)
    __new__ = _new

class Str(Constant, metaclass=_ABC):
    _fields = ('s',)
    __new__ = _new

class Bytes(Constant, metaclass=_ABC):
    _fields = ('s',)
    __new__ = _new

class NameConstant(Constant, metaclass=_ABC):
    __new__ = _new

class Ellipsis(Constant, metaclass=_ABC):
    _fields = ()

    def __new__(cls, *args, **kwargs):
        if cls is Ellipsis:
            return Constant(..., *args, **kwargs)
        return Constant.__new__(cls, *args, **kwargs)

_const_types = {
    Num: (int, float, complex),
    Str: (str,),
    Bytes: (bytes,),
    NameConstant: (type(None), bool),
    Ellipsis: (type(...),),
}
_const_types_not = {
    Num: (bool,),
}
_const_node_type_names = {
    bool: 'NameConstant',  # should be before int
    type(None): 'NameConstant',
    int: 'Num',
    float: 'Num',
    complex: 'Num',
    str: 'Str',
    bytes: 'Bytes',
    type(...): 'Ellipsis',
}

# Large float and imaginary literals get turned into infinities in the AST.
# We unparse those infinities to INFSTR.
_INFSTR = "1e" + repr(sys.float_info.max_10_exp + 1)

class _Unparser(NodeVisitor):
    """Methods in this class recursively traverse an AST and
    output source code for the abstract syntax; original formatting
    is disregarded."""

    def __init__(self):
        self._source = []
        self._buffer = []
        self._indent = 0

    def interleave(self, inter, f, seq):
        """Call f on each item in seq, calling inter() in between."""
        seq = iter(seq)
        try:
            f(next(seq))
        except StopIteration:
            pass
        else:
            for x in seq:
                inter()
                f(x)

    def fill(self, text=""):
        """Indent a piece of text and append it, according to the current
        indentation level"""
        self.write("\n" + "    " * self._indent + text)

    def write(self, text):
        """Append a piece of text"""
        self._source.append(text)

    def buffer_writer(self, text):
        self._buffer.append(text)

    @property
    def buffer(self):
        value = "".join(self._buffer)
        self._buffer.clear()
        return value

    class _Block:
        """A context manager for preparing the source for blocks. It adds
        the character':', increases the indentation on enter and decreases
        the indentation on exit."""
        def __init__(self, unparser):
            self.unparser = unparser

        def __enter__(self):
            self.unparser.write(":")
            self.unparser._indent += 1

        def __exit__(self, exc_type, exc_value, traceback):
            self.unparser._indent -= 1

    def block(self):
        return self._Block(self)

    def traverse(self, node):
        if isinstance(node, list):
            for item in node:
                self.traverse(item)
        else:
            super().visit(node)

    def visit(self, node):
        """Outputs a source code string that, if converted back to an ast
        (using ast.parse) will generate an AST equivalent to *node*"""
        self._source = []
        self.traverse(node)
        return "".join(self._source)

    def visit_Module(self, node):
        for subnode in node.body:
            self.traverse(subnode)

    def visit_Expr(self, node):
        self.fill()
        self.traverse(node.value)

    def visit_NamedExpr(self, node):
        self.write("(")
        self.traverse(node.target)
        self.write(" := ")
        self.traverse(node.value)
        self.write(")")

    def visit_Import(self, node):
        self.fill("import ")
        self.interleave(lambda: self.write(", "), self.traverse, node.names)

    def visit_ImportFrom(self, node):
        self.fill("from ")
        self.write("." * node.level)
        if node.module:
            self.write(node.module)
        self.write(" import ")
        self.interleave(lambda: self.write(", "), self.traverse, node.names)

    def visit_Assign(self, node):
        self.fill()
        for target in node.targets:
            self.traverse(target)
            self.write(" = ")
        self.traverse(node.value)

    def visit_AugAssign(self, node):
        self.fill()
        self.traverse(node.target)
        self.write(" " + self.binop[node.op.__class__.__name__] + "= ")
        self.traverse(node.value)

    def visit_AnnAssign(self, node):
        self.fill()
        if not node.simple and isinstance(node.target, Name):
            self.write("(")
        self.traverse(node.target)
        if not node.simple and isinstance(node.target, Name):
            self.write(")")
        self.write(": ")
        self.traverse(node.annotation)
        if node.value:
            self.write(" = ")
            self.traverse(node.value)

    def visit_Return(self, node):
        self.fill("return")
        if node.value:
            self.write(" ")
            self.traverse(node.value)

    def visit_Pass(self, node):
        self.fill("pass")

    def visit_Break(self, node):
        self.fill("break")

    def visit_Continue(self, node):
        self.fill("continue")

    def visit_Delete(self, node):
        self.fill("del ")
        self.interleave(lambda: self.write(", "), self.traverse, node.targets)

    def visit_Assert(self, node):
        self.fill("assert ")
        self.traverse(node.test)
        if node.msg:
            self.write(", ")
            self.traverse(node.msg)

    def visit_Global(self, node):
        self.fill("global ")
        self.interleave(lambda: self.write(", "), self.write, node.names)

    def visit_Nonlocal(self, node):
        self.fill("nonlocal ")
        self.interleave(lambda: self.write(", "), self.write, node.names)

    def visit_Await(self, node):
        self.write("(")
        self.write("await")
        if node.value:
            self.write(" ")
            self.traverse(node.value)
        self.write(")")

    def visit_Yield(self, node):
        self.write("(")
        self.write("yield")
        if node.value:
            self.write(" ")
            self.traverse(node.value)
        self.write(")")

    def visit_YieldFrom(self, node):
        self.write("(")
        self.write("yield from")
        if node.value:
            self.write(" ")
            self.traverse(node.value)
        self.write(")")

    def visit_Raise(self, node):
        self.fill("raise")
        if not node.exc:
            if node.cause:
                raise ValueError(f"Node can't use cause without an exception.")
            return
        self.write(" ")
        self.traverse(node.exc)
        if node.cause:
            self.write(" from ")
            self.traverse(node.cause)

    def visit_Try(self, node):
        self.fill("try")
        with self.block():
            self.traverse(node.body)
        for ex in node.handlers:
            self.traverse(ex)
        if node.orelse:
            self.fill("else")
            with self.block():
                self.traverse(node.orelse)
        if node.finalbody:
            self.fill("finally")
            with self.block():
                self.traverse(node.finalbody)

    def visit_ExceptHandler(self, node):
        self.fill("except")
        if node.type:
            self.write(" ")
            self.traverse(node.type)
        if node.name:
            self.write(" as ")
            self.write(node.name)
        with self.block():
            self.traverse(node.body)

    def visit_ClassDef(self, node):
        self.write("\n")
        for deco in node.decorator_list:
            self.fill("@")
            self.traverse(deco)
        self.fill("class " + node.name)
        self.write("(")
        comma = False
        for e in node.bases:
            if comma:
                self.write(", ")
            else:
                comma = True
            self.traverse(e)
        for e in node.keywords:
            if comma:
                self.write(", ")
            else:
                comma = True
            self.traverse(e)
        self.write(")")

        with self.block():
            self.traverse(node.body)

    def visit_FunctionDef(self, node):
        self.__FunctionDef_helper(node, "def")

    def visit_AsyncFunctionDef(self, node):
        self.__FunctionDef_helper(node, "async def")

    def __FunctionDef_helper(self, node, fill_suffix):
        self.write("\n")
        for deco in node.decorator_list:
            self.fill("@")
            self.traverse(deco)
        def_str = fill_suffix + " " + node.name + "("
        self.fill(def_str)
        self.traverse(node.args)
        self.write(")")
        if node.returns:
            self.write(" -> ")
            self.traverse(node.returns)
        with self.block():
            self.traverse(node.body)

    def visit_For(self, node):
        self.__For_helper("for ", node)

    def visit_AsyncFor(self, node):
        self.__For_helper("async for ", node)

    def __For_helper(self, fill, node):
        self.fill(fill)
        self.traverse(node.target)
        self.write(" in ")
        self.traverse(node.iter)
        with self.block():
            self.traverse(node.body)
        if node.orelse:
            self.fill("else")
            with self.block():
                self.traverse(node.orelse)

    def visit_If(self, node):
        self.fill("if ")
        self.traverse(node.test)
        with self.block():
            self.traverse(node.body)
        # collapse nested ifs into equivalent elifs.
        while node.orelse and len(node.orelse) == 1 and isinstance(node.orelse[0], If):
            node = node.orelse[0]
            self.fill("elif ")
            self.traverse(node.test)
            with self.block():
                self.traverse(node.body)
        # final else
        if node.orelse:
            self.fill("else")
            with self.block():
                self.traverse(node.orelse)

    def visit_While(self, node):
        self.fill("while ")
        self.traverse(node.test)
        with self.block():
            self.traverse(node.body)
        if node.orelse:
            self.fill("else")
            with self.block():
                self.traverse(node.orelse)

    def visit_With(self, node):
        self.fill("with ")
        self.interleave(lambda: self.write(", "), self.traverse, node.items)
        with self.block():
            self.traverse(node.body)

    def visit_AsyncWith(self, node):
        self.fill("async with ")
        self.interleave(lambda: self.write(", "), self.traverse, node.items)
        with self.block():
            self.traverse(node.body)

    def visit_JoinedStr(self, node):
        self.write("f")
        self._fstring_JoinedStr(node, self.buffer_writer)
        self.write(repr(self.buffer))

    def visit_FormattedValue(self, node):
        self.write("f")
        self._fstring_FormattedValue(node, self.buffer_writer)
        self.write(repr(self.buffer))

    def _fstring_JoinedStr(self, node, write):
        for value in node.values:
            meth = getattr(self, "_fstring_" + type(value).__name__)
            meth(value, write)

    def _fstring_Constant(self, node, write):
        if not isinstance(node.value, str):
            raise ValueError("Constants inside JoinedStr should be a string.")
        value = node.value.replace("{", "{{").replace("}", "}}")
        write(value)

    def _fstring_FormattedValue(self, node, write):
        write("{")
        expr = type(self)().visit(node.value).rstrip("\n")
        if expr.startswith("{"):
            write(" ")  # Separate pair of opening brackets as "{ {"
        write(expr)
        if node.conversion != -1:
            conversion = chr(node.conversion)
            if conversion not in "sra":
                raise ValueError("Unknown f-string conversion.")
            write(f"!{conversion}")
        if node.format_spec:
            write(":")
            meth = getattr(self, "_fstring_" + type(node.format_spec).__name__)
            meth(node.format_spec, write)
        write("}")

    def visit_Name(self, node):
        self.write(node.id)

    def _write_constant(self, value):
        if isinstance(value, (float, complex)):
            # Substitute overflowing decimal literal for AST infinities.
            self.write(repr(value).replace("inf", _INFSTR))
        else:
            self.write(repr(value))

    def visit_Constant(self, node):
        value = node.value
        if isinstance(value, tuple):
            self.write("(")
            if len(value) == 1:
                self._write_constant(value[0])
                self.write(",")
            else:
                self.interleave(lambda: self.write(", "), self._write_constant, value)
            self.write(")")
        elif value is ...:
            self.write("...")
        else:
            if node.kind == "u":
                self.write("u")
            self._write_constant(node.value)

    def visit_List(self, node):
        self.write("[")
        self.interleave(lambda: self.write(", "), self.traverse, node.elts)
        self.write("]")

    def visit_ListComp(self, node):
        self.write("[")
        self.traverse(node.elt)
        for gen in node.generators:
            self.traverse(gen)
        self.write("]")

    def visit_GeneratorExp(self, node):
        self.write("(")
        self.traverse(node.elt)
        for gen in node.generators:
            self.traverse(gen)
        self.write(")")

    def visit_SetComp(self, node):
        self.write("{")
        self.traverse(node.elt)
        for gen in node.generators:
            self.traverse(gen)
        self.write("}")

    def visit_DictComp(self, node):
        self.write("{")
        self.traverse(node.key)
        self.write(": ")
        self.traverse(node.value)
        for gen in node.generators:
            self.traverse(gen)
        self.write("}")

    def visit_comprehension(self, node):
        if node.is_async:
            self.write(" async for ")
        else:
            self.write(" for ")
        self.traverse(node.target)
        self.write(" in ")
        self.traverse(node.iter)
        for if_clause in node.ifs:
            self.write(" if ")
            self.traverse(if_clause)

    def visit_IfExp(self, node):
        self.write("(")
        self.traverse(node.body)
        self.write(" if ")
        self.traverse(node.test)
        self.write(" else ")
        self.traverse(node.orelse)
        self.write(")")

    def visit_Set(self, node):
        if not node.elts:
            raise ValueError("Set node should has at least one item")
        self.write("{")
        self.interleave(lambda: self.write(", "), self.traverse, node.elts)
        self.write("}")

    def visit_Dict(self, node):
        self.write("{")

        def write_key_value_pair(k, v):
            self.traverse(k)
            self.write(": ")
            self.traverse(v)

        def write_item(item):
            k, v = item
            if k is None:
                # for dictionary unpacking operator in dicts {**{'y': 2}}
                # see PEP 448 for details
                self.write("**")
                self.traverse(v)
            else:
                write_key_value_pair(k, v)

        self.interleave(
            lambda: self.write(", "), write_item, zip(node.keys, node.values)
        )
        self.write("}")

    def visit_Tuple(self, node):
        self.write("(")
        if len(node.elts) == 1:
            elt = node.elts[0]
            self.traverse(elt)
            self.write(",")
        else:
            self.interleave(lambda: self.write(", "), self.traverse, node.elts)
        self.write(")")

    unop = {"Invert": "~", "Not": "not", "UAdd": "+", "USub": "-"}

    def visit_UnaryOp(self, node):
        self.write("(")
        self.write(self.unop[node.op.__class__.__name__])
        self.write(" ")
        self.traverse(node.operand)
        self.write(")")

    binop = {
        "Add": "+",
        "Sub": "-",
        "Mult": "*",
        "MatMult": "@",
        "Div": "/",
        "Mod": "%",
        "LShift": "<<",
        "RShift": ">>",
        "BitOr": "|",
        "BitXor": "^",
        "BitAnd": "&",
        "FloorDiv": "//",
        "Pow": "**",
    }

    def visit_BinOp(self, node):
        self.write("(")
        self.traverse(node.left)
        self.write(" " + self.binop[node.op.__class__.__name__] + " ")
        self.traverse(node.right)
        self.write(")")

    cmpops = {
        "Eq": "==",
        "NotEq": "!=",
        "Lt": "<",
        "LtE": "<=",
        "Gt": ">",
        "GtE": ">=",
        "Is": "is",
        "IsNot": "is not",
        "In": "in",
        "NotIn": "not in",
    }

    def visit_Compare(self, node):
        self.write("(")
        self.traverse(node.left)
        for o, e in zip(node.ops, node.comparators):
            self.write(" " + self.cmpops[o.__class__.__name__] + " ")
            self.traverse(e)
        self.write(")")

    boolops = {And: "and", Or: "or"}

    def visit_BoolOp(self, node):
        self.write("(")
        s = " %s " % self.boolops[node.op.__class__]
        self.interleave(lambda: self.write(s), self.traverse, node.values)
        self.write(")")

    def visit_Attribute(self, node):
        self.traverse(node.value)
        # Special case: 3.__abs__() is a syntax error, so if node.value
        # is an integer literal then we need to either parenthesize
        # it or add an extra space to get 3 .__abs__().
        if isinstance(node.value, Constant) and isinstance(node.value.value, int):
            self.write(" ")
        self.write(".")
        self.write(node.attr)

    def visit_Call(self, node):
        self.traverse(node.func)
        self.write("(")
        comma = False
        for e in node.args:
            if comma:
                self.write(", ")
            else:
                comma = True
            self.traverse(e)
        for e in node.keywords:
            if comma:
                self.write(", ")
            else:
                comma = True
            self.traverse(e)
        self.write(")")

    def visit_Subscript(self, node):
        self.traverse(node.value)
        self.write("[")
        self.traverse(node.slice)
        self.write("]")

    def visit_Starred(self, node):
        self.write("*")
        self.traverse(node.value)

    def visit_Ellipsis(self, node):
        self.write("...")

    def visit_Index(self, node):
        self.traverse(node.value)

    def visit_Slice(self, node):
        if node.lower:
            self.traverse(node.lower)
        self.write(":")
        if node.upper:
            self.traverse(node.upper)
        if node.step:
            self.write(":")
            self.traverse(node.step)

    def visit_ExtSlice(self, node):
        self.interleave(lambda: self.write(", "), self.traverse, node.dims)

    def visit_arg(self, node):
        self.write(node.arg)
        if node.annotation:
            self.write(": ")
            self.traverse(node.annotation)

    def visit_arguments(self, node):
        first = True
        # normal arguments
        all_args = node.posonlyargs + node.args
        defaults = [None] * (len(all_args) - len(node.defaults)) + node.defaults
        for index, elements in enumerate(zip(all_args, defaults), 1):
            a, d = elements
            if first:
                first = False
            else:
                self.write(", ")
            self.traverse(a)
            if d:
                self.write("=")
                self.traverse(d)
            if index == len(node.posonlyargs):
                self.write(", /")

        # varargs, or bare '*' if no varargs but keyword-only arguments present
        if node.vararg or node.kwonlyargs:
            if first:
                first = False
            else:
                self.write(", ")
            self.write("*")
            if node.vararg:
                self.write(node.vararg.arg)
                if node.vararg.annotation:
                    self.write(": ")
                    self.traverse(node.vararg.annotation)

        # keyword-only arguments
        if node.kwonlyargs:
            for a, d in zip(node.kwonlyargs, node.kw_defaults):
                self.write(", ")
                self.traverse(a)
                if d:
                    self.write("=")
                    self.traverse(d)

        # kwargs
        if node.kwarg:
            if first:
                first = False
            else:
                self.write(", ")
            self.write("**" + node.kwarg.arg)
            if node.kwarg.annotation:
                self.write(": ")
                self.traverse(node.kwarg.annotation)

    def visit_keyword(self, node):
        if node.arg is None:
            self.write("**")
        else:
            self.write(node.arg)
            self.write("=")
        self.traverse(node.value)

    def visit_Lambda(self, node):
        self.write("(")
        self.write("lambda ")
        self.traverse(node.args)
        self.write(": ")
        self.traverse(node.body)
        self.write(")")

    def visit_alias(self, node):
        self.write(node.name)
        if node.asname:
            self.write(" as " + node.asname)

    def visit_withitem(self, node):
        self.traverse(node.context_expr)
        if node.optional_vars:
            self.write(" as ")
            self.traverse(node.optional_vars)

def unparse(ast_obj):
    unparser = _Unparser()
    return unparser.visit(ast_obj)


def main():
    import argparse

    parser = argparse.ArgumentParser(prog='python -m ast')
    parser.add_argument('infile', type=argparse.FileType(mode='rb'), nargs='?',
                        default='-',
                        help='the file to parse; defaults to stdin')
    parser.add_argument('-m', '--mode', default='exec',
                        choices=('exec', 'single', 'eval', 'func_type'),
                        help='specify what kind of code must be parsed')
    parser.add_argument('-a', '--include-attributes', action='store_true',
                        help='include attributes such as line numbers and '
                             'column offsets')
    args = parser.parse_args()

    with args.infile as infile:
        source = infile.read()
    tree = parse(source, args.infile.name, args.mode, type_comments=True)
    print(dump(tree, include_attributes=args.include_attributes, indent=3))

if __name__ == '__main__':
    main()