Class GraphBuilder

tensorflow/python/autograph/pyct/cfg.py:231–610

Builder that constructs a CFG from a given AST. This GraphBuilder facilitates constructing the DAG that forms the CFG when nodes are supplied in lexical order (i.e., top-down, depth first). Under these conditions, it supports building patterns found in typical structured programs. This

class GraphBuilder(object):
  """Builder that constructs a CFG from a given AST.

  This GraphBuilder facilitates constructing the DAG that forms the CFG when
  nodes
  are supplied in lexical order (i.e., top-down, depth first). Under these
  conditions, it supports building patterns found in typical structured
  programs.

  This builder ignores the flow generated by exceptions, which are assumed to
  always be catastrophic and present purely for diagnostic purposes (e.g. to
  print debug information). Statements like raise and try/catch sections are
  allowed and will generate control flow edges, but ordinaty statements are
  assumed not to raise exceptions.

  Finally sections are also correctly interleaved between break/continue/return
  nodes and their subsequent statements.

  Important concepts:
   * nodes - nodes refer refer to CFG nodes; AST nodes are qualified explicitly
   * leaf set - since the graph is constructed gradually, a leaf set maintains
     the CFG nodes that will precede the node that the builder expects to
     receive next; when an ordinary node is added, it is connected to the
     existing leaves and it in turn becomes the new leaf
   * jump nodes - nodes that should generate edges other than what
     ordinary nodes would; these correspond to break, continue and return
     statements
   * sections - logical delimiters for subgraphs that require special
     edges; there are various types of nodes, each admitting various
     types of jump nodes; sections are identified by their corresponding AST
     node
  """

  # TODO(mdan): Perhaps detail this in a markdown doc.
  # TODO(mdan): Add exception support.

  def __init__(self, parent_ast_node):
    self.reset()
    self.parent = parent_ast_node

  def reset(self):
    """Resets the state of this factory."""
    self.head = None
    self.errors = set()
    self.node_index = {}

    # TODO(mdan): Too many primitives. Use classes.
    self.leaves = set()

    # Note: This mechanism requires that nodes are added in lexical order (top
    # to bottom, depth first).
    self.active_stmts = set()
    self.owners = {}  # type: Set[any]
    self.forward_edges = set()  # type: Tuple[Node, Node] # (from, to)

    self.finally_sections = {}
    # Dict values represent (entry, exits)
    self.finally_section_subgraphs = {