return "..." rather than raising RecursionError for repr() on a cycli…

…c object

Fixes #95

changelog.d/95.change.rst

@@ -0,0 +1 @@
+``x=X(); x.cycle = x; repr(x)`` will no longer raise a ``RecursionError``, and will instead show as ``X(x=...)``.

src/attr/_make.py

@@ -3,6 +3,7 @@
 import hashlib
 import linecache
 import sys
+import threading
 import warnings
 
 from operator import itemgetter
@@ -953,6 +954,9 @@ def _add_cmp(cls, attrs=None):
  return cls
 
 
+_already_repring = threading.local()
+
+
 def _make_repr(attrs, ns):
  """
  Make a repr method for *attr_names* adding *ns* to the full name.
@@ -967,6 +971,14 @@ def __repr__(self):
  """
  Automatically created by attrs.
  """
+ try:
+ working_set = _already_repring.working_set
+ except AttributeError:
+ working_set = set()
+ _already_repring.working_set = working_set
+
+ if id(self) in working_set:
+ return "..."
  real_cls = self.__class__
  if ns is None:
  qualname = getattr(real_cls, "__qualname__", None)
@@ -977,13 +989,23 @@ def __repr__(self):
  else:
  class_name = ns + "." + real_cls.__name__
 
- return "{0}({1})".format(
- class_name,
- ", ".join(
- name + "=" + repr(getattr(self, name, NOTHING))
- for name in attr_names
- )
- )
+ # Since 'self' remains on the stack (i.e.: strongly referenced) for the
+ # duration of this call, it's safe to depend on id(...) stability, and
+ # not need to track the instance and therefore worry about properties
+ # like weakref- or hash-ability.
+ working_set.add(id(self))
+ try:
+ result = [class_name, "("]
+ first = True
+ for name in attr_names:
+ if first:
+ first = False
+ else:
+ result.append(", ")
+ result.extend((name, "=", repr(getattr(self, name, NOTHING))))
+ return "".join(result) + ")"
+ finally:
+ working_set.remove(id(self))
  return __repr__
 
 

tests/test_dunders.py

@@ -187,6 +187,20 @@ def test_repr_works(self, cls):
  """
  assert "C(a=1, b=2)" == repr(cls(1, 2))
 
+ def test_infinite_recursion(self):
+ """
+ In the presence of a cyclic graph, repr will emit an ellipsis and not
+ raise an exception.
+ """
+ @attr.s
+ class Cycle(object):
+ value = attr.ib(default=7)
+ cycle = attr.ib(default=None)
+
+ cycle = Cycle()
+ cycle.cycle = cycle
+ assert "Cycle(value=7, cycle=...)" == repr(cycle)
+
  def test_underscores(self):
  """
  repr does not strip underscores.