Merge pull request #3163 from rkern/fix-empty-frozen-set-pretty

Fix the set and frozenset pretty printer to handle the empty case correctly

IPython/lib/pretty.py

@@ -1,742 +1,776
 # -*- coding: utf-8 -*-
 """
 Python advanced pretty printer.  This pretty printer is intended to
 replace the old `pprint` python module which does not allow developers
 to provide their own pretty print callbacks.
 
 This module is based on ruby's `prettyprint.rb` library by `Tanaka Akira`.
 
 
 Example Usage
 -------------
 
 To directly print the representation of an object use `pprint`::
 
     from pretty import pprint
     pprint(complex_object)
 
 To get a string of the output use `pretty`::
 
     from pretty import pretty
     string = pretty(complex_object)
 
 
 Extending
 ---------
 
 The pretty library allows developers to add pretty printing rules for their
 own objects.  This process is straightforward.  All you have to do is to
 add a `_repr_pretty_` method to your object and call the methods on the
 pretty printer passed::
 
     class MyObject(object):
 
         def _repr_pretty_(self, p, cycle):
             ...
 
 Depending on the python version you want to support you have two
 possibilities.  The following list shows the python 2.5 version and the
 compatibility one.
 
 
 Here the example implementation of a `_repr_pretty_` method for a list
 subclass for python 2.5 and higher (python 2.5 requires the with statement
 __future__ import)::
 
     class MyList(list):
 
         def _repr_pretty_(self, p, cycle):
             if cycle:
                 p.text('MyList(...)')
             else:
                 with p.group(8, 'MyList([', '])'):
                     for idx, item in enumerate(self):
                         if idx:
                             p.text(',')
                             p.breakable()
                         p.pretty(item)
 
 The `cycle` parameter is `True` if pretty detected a cycle.  You *have* to
 react to that or the result is an infinite loop.  `p.text()` just adds
 non breaking text to the output, `p.breakable()` either adds a whitespace
 or breaks here.  If you pass it an argument it's used instead of the
 default space.  `p.pretty` prettyprints another object using the pretty print
 method.
 
 The first parameter to the `group` function specifies the extra indentation
 of the next line.  In this example the next item will either be not
 breaked (if the items are short enough) or aligned with the right edge of
 the opening bracked of `MyList`.
 
 If you want to support python 2.4 and lower you can use this code::
 
     class MyList(list):
 
         def _repr_pretty_(self, p, cycle):
             if cycle:
                 p.text('MyList(...)')
             else:
                 p.begin_group(8, 'MyList([')
                 for idx, item in enumerate(self):
                     if idx:
                         p.text(',')
                         p.breakable()
                     p.pretty(item)
                 p.end_group(8, '])')
 
 If you just want to indent something you can use the group function
 without open / close parameters.  Under python 2.5 you can also use this
 code::
 
     with p.indent(2):
         ...
 
 Or under python2.4 you might want to modify ``p.indentation`` by hand but
 this is rather ugly.
 
 Inheritance diagram:
 
 .. inheritance-diagram:: IPython.lib.pretty
    :parts: 3
 
 :copyright: 2007 by Armin Ronacher.
             Portions (c) 2009 by Robert Kern.
 :license: BSD License.
 """
 from __future__ import with_statement
 from contextlib import contextmanager
 import sys
 import types
 import re
 import datetime
 from StringIO import StringIO
 from collections import deque
 
 
 __all__ = ['pretty', 'pprint', 'PrettyPrinter', 'RepresentationPrinter',
     'for_type', 'for_type_by_name']
 
 
 _re_pattern_type = type(re.compile(''))
 
 
 def pretty(obj, verbose=False, max_width=79, newline='\n'):
     """
     Pretty print the object's representation.
     """
     stream = StringIO()
     printer = RepresentationPrinter(stream, verbose, max_width, newline)
     printer.pretty(obj)
     printer.flush()
     return stream.getvalue()
 
 
 def pprint(obj, verbose=False, max_width=79, newline='\n'):
     """
     Like `pretty` but print to stdout.
     """
     printer = RepresentationPrinter(sys.stdout, verbose, max_width, newline)
     printer.pretty(obj)
     printer.flush()
     sys.stdout.write(newline)
     sys.stdout.flush()
 
 class _PrettyPrinterBase(object):
 
     @contextmanager
     def indent(self, indent):
         """with statement support for indenting/dedenting."""
         self.indentation += indent
         try:
             yield
         finally:
             self.indentation -= indent
 
     @contextmanager
     def group(self, indent=0, open='', close=''):
         """like begin_group / end_group but for the with statement."""
         self.begin_group(indent, open)
         try:
             yield
         finally:
             self.end_group(indent, close)
 
 class PrettyPrinter(_PrettyPrinterBase):
     """
     Baseclass for the `RepresentationPrinter` prettyprinter that is used to
     generate pretty reprs of objects.  Contrary to the `RepresentationPrinter`
     this printer knows nothing about the default pprinters or the `_repr_pretty_`
     callback method.
     """
 
     def __init__(self, output, max_width=79, newline='\n'):
         self.output = output
         self.max_width = max_width
         self.newline = newline
         self.output_width = 0
         self.buffer_width = 0
         self.buffer = deque()
 
         root_group = Group(0)
         self.group_stack = [root_group]
         self.group_queue = GroupQueue(root_group)
         self.indentation = 0
 
     def _break_outer_groups(self):
         while self.max_width < self.output_width + self.buffer_width:
             group = self.group_queue.deq()
             if not group:
                 return
             while group.breakables:
                 x = self.buffer.popleft()
                 self.output_width = x.output(self.output, self.output_width)
                 self.buffer_width -= x.width
             while self.buffer and isinstance(self.buffer[0], Text):
                 x = self.buffer.popleft()
                 self.output_width = x.output(self.output, self.output_width)
                 self.buffer_width -= x.width
 
     def text(self, obj):
         """Add literal text to the output."""
         width = len(obj)
         if self.buffer:
             text = self.buffer[-1]
             if not isinstance(text, Text):
                 text = Text()
                 self.buffer.append(text)
             text.add(obj, width)
             self.buffer_width += width
             self._break_outer_groups()
         else:
             self.output.write(obj)
             self.output_width += width
 
     def breakable(self, sep=' '):
         """
         Add a breakable separator to the output.  This does not mean that it
         will automatically break here.  If no breaking on this position takes
         place the `sep` is inserted which default to one space.
         """
         width = len(sep)
         group = self.group_stack[-1]
         if group.want_break:
             self.flush()
             self.output.write(self.newline)
             self.output.write(' ' * self.indentation)
             self.output_width = self.indentation
             self.buffer_width = 0
         else:
             self.buffer.append(Breakable(sep, width, self))
             self.buffer_width += width
             self._break_outer_groups()
 
 
     def begin_group(self, indent=0, open=''):
         """
         Begin a group.  If you want support for python < 2.5 which doesn't has
         the with statement this is the preferred way:
 
             p.begin_group(1, '{')
             ...
             p.end_group(1, '}')
 
         The python 2.5 expression would be this:
 
             with p.group(1, '{', '}'):
                 ...
 
         The first parameter specifies the indentation for the next line (usually
         the width of the opening text), the second the opening text.  All
         parameters are optional.
         """
         if open:
             self.text(open)
         group = Group(self.group_stack[-1].depth + 1)
         self.group_stack.append(group)
         self.group_queue.enq(group)
         self.indentation += indent
 
     def end_group(self, dedent=0, close=''):
         """End a group. See `begin_group` for more details."""
         self.indentation -= dedent
         group = self.group_stack.pop()
         if not group.breakables:
             self.group_queue.remove(group)
         if close:
             self.text(close)
 
     def flush(self):
         """Flush data that is left in the buffer."""
         for data in self.buffer:
             self.output_width += data.output(self.output, self.output_width)
         self.buffer.clear()
         self.buffer_width = 0
 
 
 def _get_mro(obj_class):
     """ Get a reasonable method resolution order of a class and its superclasses
     for both old-style and new-style classes.
     """
     if not hasattr(obj_class, '__mro__'):
         # Old-style class. Mix in object to make a fake new-style class.
         try:
             obj_class = type(obj_class.__name__, (obj_class, object), {})
         except TypeError:
             # Old-style extension type that does not descend from object.
             # FIXME: try to construct a more thorough MRO.
             mro = [obj_class]
         else:
             mro = obj_class.__mro__[1:-1]
     else:
         mro = obj_class.__mro__
     return mro
 
 
 class RepresentationPrinter(PrettyPrinter):
     """
     Special pretty printer that has a `pretty` method that calls the pretty
     printer for a python object.
 
     This class stores processing data on `self` so you must *never* use
     this class in a threaded environment.  Always lock it or reinstanciate
     it.
 
     Instances also have a verbose flag callbacks can access to control their
     output.  For example the default instance repr prints all attributes and
     methods that are not prefixed by an underscore if the printer is in
     verbose mode.
     """
 
     def __init__(self, output, verbose=False, max_width=79, newline='\n',
         singleton_pprinters=None, type_pprinters=None, deferred_pprinters=None):
 
         PrettyPrinter.__init__(self, output, max_width, newline)
         self.verbose = verbose
         self.stack = []
         if singleton_pprinters is None:
             singleton_pprinters = _singleton_pprinters.copy()
         self.singleton_pprinters = singleton_pprinters
         if type_pprinters is None:
             type_pprinters = _type_pprinters.copy()
         self.type_pprinters = type_pprinters
         if deferred_pprinters is None:
             deferred_pprinters = _deferred_type_pprinters.copy()
         self.deferred_pprinters = deferred_pprinters
 
     def pretty(self, obj):
         """Pretty print the given object."""
         obj_id = id(obj)
         cycle = obj_id in self.stack
         self.stack.append(obj_id)
         self.begin_group()
         try:
             obj_class = getattr(obj, '__class__', None) or type(obj)
             # First try to find registered singleton printers for the type.
             try:
                 printer = self.singleton_pprinters[obj_id]
             except (TypeError, KeyError):
                 pass
             else:
                 return printer(obj, self, cycle)
             # Next walk the mro and check for either:
             #   1) a registered printer
             #   2) a _repr_pretty_ method
             for cls in _get_mro(obj_class):
                 if cls in self.type_pprinters:
                     # printer registered in self.type_pprinters
                     return self.type_pprinters[cls](obj, self, cycle)
                 else:
                     # deferred printer
                     printer = self._in_deferred_types(cls)
                     if printer is not None:
                         return printer(obj, self, cycle)
                     else:
                         # Finally look for special method names.
                         # Some objects automatically create any requested
                         # attribute. Try to ignore most of them by checking for
                         # callability.
                         if '_repr_pretty_' in cls.__dict__:
                             meth = cls._repr_pretty_
                             if callable(meth):
                                 return meth(obj, self, cycle)
             return _default_pprint(obj, self, cycle)
         finally:
             self.end_group()
             self.stack.pop()
 
     def _in_deferred_types(self, cls):
         """
         Check if the given class is specified in the deferred type registry.
 
         Returns the printer from the registry if it exists, and None if the
         class is not in the registry. Successful matches will be moved to the
         regular type registry for future use.
         """
         mod = getattr(cls, '__module__', None)
         name = getattr(cls, '__name__', None)
         key = (mod, name)
         printer = None
         if key in self.deferred_pprinters:
             # Move the printer over to the regular registry.
             printer = self.deferred_pprinters.pop(key)
             self.type_pprinters[cls] = printer
         return printer
 
 
 class Printable(object):
 
     def output(self, stream, output_width):
         return output_width
 
 
 class Text(Printable):
 
     def __init__(self):
         self.objs = []
         self.width = 0
 
     def output(self, stream, output_width):
         for obj in self.objs:
             stream.write(obj)
         return output_width + self.width
 
     def add(self, obj, width):
         self.objs.append(obj)
         self.width += width
 
 
 class Breakable(Printable):
 
     def __init__(self, seq, width, pretty):
         self.obj = seq
         self.width = width
         self.pretty = pretty
         self.indentation = pretty.indentation
         self.group = pretty.group_stack[-1]
         self.group.breakables.append(self)
 
     def output(self, stream, output_width):
         self.group.breakables.popleft()
         if self.group.want_break:
             stream.write(self.pretty.newline)
             stream.write(' ' * self.indentation)
             return self.indentation
         if not self.group.breakables:
             self.pretty.group_queue.remove(self.group)
         stream.write(self.obj)
         return output_width + self.width
 
 
 class Group(Printable):
 
     def __init__(self, depth):
         self.depth = depth
         self.breakables = deque()
         self.want_break = False
 
 
 class GroupQueue(object):
 
     def __init__(self, *groups):
         self.queue = []
         for group in groups:
             self.enq(group)
 
     def enq(self, group):
         depth = group.depth
         while depth > len(self.queue) - 1:
             self.queue.append([])
         self.queue[depth].append(group)
 
     def deq(self):
         for stack in self.queue:
             for idx, group in enumerate(reversed(stack)):
                 if group.breakables:
                     del stack[idx]
                     group.want_break = True
                     return group
             for group in stack:
                 group.want_break = True
             del stack[:]
 
     def remove(self, group):
         try:
             self.queue[group.depth].remove(group)
         except ValueError:
             pass
 
 try:
     _baseclass_reprs = (object.__repr__, types.InstanceType.__repr__)
 except AttributeError:  # Python 3
     _baseclass_reprs = (object.__repr__,)
 
 
 def _default_pprint(obj, p, cycle):
     """
     The default print function.  Used if an object does not provide one and
     it's none of the builtin objects.
     """
     klass = getattr(obj, '__class__', None) or type(obj)
     if getattr(klass, '__repr__', None) not in _baseclass_reprs:
         # A user-provided repr.
         p.text(repr(obj))
         return
     p.begin_group(1, '<')
     p.pretty(klass)
     p.text(' at 0x%x' % id(obj))
     if cycle:
         p.text(' ...')
     elif p.verbose:
         first = True
         for key in dir(obj):
             if not key.startswith('_'):
                 try:
                     value = getattr(obj, key)
                 except AttributeError:
                     continue
                 if isinstance(value, types.MethodType):
                     continue
                 if not first:
                     p.text(',')
                 p.breakable()
                 p.text(key)
                 p.text('=')
                 step = len(key) + 1
                 p.indentation += step
                 p.pretty(value)
                 p.indentation -= step
                 first = False
     p.end_group(1, '>')
 
 
 def _seq_pprinter_factory(start, end, basetype):
     """
     Factory that returns a pprint function useful for sequences.  Used by
-    the default pprint for tuples, dicts, lists, sets and frozensets.
+    the default pprint for tuples, dicts, and lists.
     """
     def inner(obj, p, cycle):
         typ = type(obj)
         if basetype is not None and typ is not basetype and typ.__repr__ != basetype.__repr__:
             # If the subclass provides its own repr, use it instead.
             return p.text(typ.__repr__(obj))
 
         if cycle:
             return p.text(start + '...' + end)
         step = len(start)
         p.begin_group(step, start)
         for idx, x in enumerate(obj):
             if idx:
                 p.text(',')
                 p.breakable()
             p.pretty(x)
         if len(obj) == 1 and type(obj) is tuple:
             # Special case for 1-item tuples.
             p.text(',')
         p.end_group(step, end)
     return inner
 
 
+def _set_pprinter_factory(start, end, basetype):
+    """
+    Factory that returns a pprint function useful for sets and frozensets.
+    """
+    def inner(obj, p, cycle):
+        typ = type(obj)
+        if basetype is not None and typ is not basetype and typ.__repr__ != basetype.__repr__:
+            # If the subclass provides its own repr, use it instead.
+            return p.text(typ.__repr__(obj))
+
+        if cycle:
+            return p.text(start + '...' + end)
+        if len(obj) == 0:
+            # Special case.
+            p.text(basetype.__name__ + '()')
+        else:
+            step = len(start)
+            p.begin_group(step, start)
+            # Like dictionary keys, we will try to sort the items.
+            items = list(obj)
+            try:
+                items.sort()
+            except Exception:
+                # Sometimes the items don't sort.
+                pass
+            for idx, x in enumerate(items):
+                if idx:
+                    p.text(',')
+                    p.breakable()
+                p.pretty(x)
+            p.end_group(step, end)
+    return inner
+
+
 def _dict_pprinter_factory(start, end, basetype=None):
     """
     Factory that returns a pprint function used by the default pprint of
     dicts and dict proxies.
     """
     def inner(obj, p, cycle):
         typ = type(obj)
         if basetype is not None and typ is not basetype and typ.__repr__ != basetype.__repr__:
             # If the subclass provides its own repr, use it instead.
             return p.text(typ.__repr__(obj))
 
         if cycle:
             return p.text('{...}')
         p.begin_group(1, start)
         keys = obj.keys()
         try:
             keys.sort()
         except Exception as e:
             # Sometimes the keys don't sort.
             pass
         for idx, key in enumerate(keys):
             if idx:
                 p.text(',')
                 p.breakable()
             p.pretty(key)
             p.text(': ')
             p.pretty(obj[key])
         p.end_group(1, end)
     return inner
 
 
 def _super_pprint(obj, p, cycle):
     """The pprint for the super type."""
     p.begin_group(8, '<super: ')
     p.pretty(obj.__self_class__)
     p.text(',')
     p.breakable()
     p.pretty(obj.__self__)
     p.end_group(8, '>')
 
 
 def _re_pattern_pprint(obj, p, cycle):
     """The pprint function for regular expression patterns."""
     p.text('re.compile(')
     pattern = repr(obj.pattern)
     if pattern[:1] in 'uU':
         pattern = pattern[1:]
         prefix = 'ur'
     else:
         prefix = 'r'
     pattern = prefix + pattern.replace('\\\\', '\\')
     p.text(pattern)
     if obj.flags:
         p.text(',')
         p.breakable()
         done_one = False
         for flag in ('TEMPLATE', 'IGNORECASE', 'LOCALE', 'MULTILINE', 'DOTALL',
             'UNICODE', 'VERBOSE', 'DEBUG'):
             if obj.flags & getattr(re, flag):
                 if done_one:
                     p.text('|')
                 p.text('re.' + flag)
                 done_one = True
     p.text(')')
 
 
 def _type_pprint(obj, p, cycle):
     """The pprint for classes and types."""
     try:
         mod = obj.__module__
     except AttributeError:
         # Heap allocated types might not have the module attribute.
         return p.text(obj.__name__)
 
     if mod in ('__builtin__', 'exceptions'):
         name = obj.__name__
     else:
         name = mod + '.' + obj.__name__
     p.text(name)
 
 
 def _repr_pprint(obj, p, cycle):
     """A pprint that just redirects to the normal repr function."""
     p.text(repr(obj))
 
 
 def _function_pprint(obj, p, cycle):
     """Base pprint for all functions and builtin functions."""
     if obj.__module__ in ('__builtin__', 'exceptions') or not obj.__module__:
         name = obj.__name__
     else:
         name = obj.__module__ + '.' + obj.__name__
     p.text('<function %s>' % name)
 
 
 def _exception_pprint(obj, p, cycle):
     """Base pprint for all exceptions."""
     if obj.__class__.__module__ in ('exceptions', 'builtins'):
         name = obj.__class__.__name__
     else:
         name = '%s.%s' % (
             obj.__class__.__module__,
             obj.__class__.__name__
         )
     step = len(name) + 1
     p.begin_group(step, name + '(')
     for idx, arg in enumerate(getattr(obj, 'args', ())):
         if idx:
             p.text(',')
             p.breakable()
         p.pretty(arg)
     p.end_group(step, ')')
 
 
 #: the exception base
 try:
     _exception_base = BaseException
 except NameError:
     _exception_base = Exception
 
 
 #: printers for builtin types
 _type_pprinters = {
     int:                        _repr_pprint,
     long:                       _repr_pprint,
     float:                      _repr_pprint,
     str:                        _repr_pprint,
     unicode:                    _repr_pprint,
     tuple:                      _seq_pprinter_factory('(', ')', tuple),
     list:                       _seq_pprinter_factory('[', ']', list),
     dict:                       _dict_pprinter_factory('{', '}', dict),
     
-    set:                        _seq_pprinter_factory('{', '}', set),
-    frozenset:                  _seq_pprinter_factory('frozenset([', '])', frozenset),
+    set:                        _set_pprinter_factory('{', '}', set),
+    frozenset:                  _set_pprinter_factory('frozenset({', '})', frozenset),
     super:                      _super_pprint,
     _re_pattern_type:           _re_pattern_pprint,
     type:                       _type_pprint,
     types.FunctionType:         _function_pprint,
     types.BuiltinFunctionType:  _function_pprint,
     types.SliceType:            _repr_pprint,
     types.MethodType:           _repr_pprint,
     
     datetime.datetime:          _repr_pprint,
     datetime.timedelta:         _repr_pprint,
     _exception_base:            _exception_pprint
 }
 
 try:
     _type_pprinters[types.DictProxyType] = _dict_pprinter_factory('<dictproxy {', '}>')
     _type_pprinters[types.ClassType] = _type_pprint
 except AttributeError: # Python 3
     pass
     
 try:
     _type_pprinters[xrange] = _repr_pprint
 except NameError:
     _type_pprinters[range] = _repr_pprint
 
 #: printers for types specified by name
 _deferred_type_pprinters = {
 }
 
 def for_type(typ, func):
     """
     Add a pretty printer for a given type.
     """
     oldfunc = _type_pprinters.get(typ, None)
     if func is not None:
         # To support easy restoration of old pprinters, we need to ignore Nones.
         _type_pprinters[typ] = func
     return oldfunc
 
 def for_type_by_name(type_module, type_name, func):
     """
     Add a pretty printer for a type specified by the module and name of a type
     rather than the type object itself.
     """
     key = (type_module, type_name)
     oldfunc = _deferred_type_pprinters.get(key, None)
     if func is not None:
         # To support easy restoration of old pprinters, we need to ignore Nones.
         _deferred_type_pprinters[key] = func
     return oldfunc
 
 
 #: printers for the default singletons
 _singleton_pprinters = dict.fromkeys(map(id, [None, True, False, Ellipsis,
                                       NotImplemented]), _repr_pprint)
 
 
 if __name__ == '__main__':
     from random import randrange
     class Foo(object):
         def __init__(self):
             self.foo = 1
             self.bar = re.compile(r'\s+')
             self.blub = dict.fromkeys(range(30), randrange(1, 40))
             self.hehe = 23424.234234
             self.list = ["blub", "blah", self]
 
         def get_foo(self):
             print "foo"
 
     pprint(Foo(), verbose=True)

IPython/lib/tests/test_pretty.py

@@ -1,94 +1,108
 """Tests for IPython.lib.pretty.
 """
 #-----------------------------------------------------------------------------
 # Copyright (c) 2011, the IPython Development Team.
 #
 # Distributed under the terms of the Modified BSD License.
 #
 # The full license is in the file COPYING.txt, distributed with this software.
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 from __future__ import print_function
 
 # Third-party imports
 import nose.tools as nt
 
 # Our own imports
 from IPython.lib import pretty
 from IPython.testing.decorators import skip_without
 
 #-----------------------------------------------------------------------------
 # Classes and functions
 #-----------------------------------------------------------------------------
 
 class MyList(object):
     def __init__(self, content):
         self.content = content
     def _repr_pretty_(self, p, cycle):
         if cycle:
             p.text("MyList(...)")
         else:
             with p.group(3, "MyList(", ")"):
                 for (i, child) in enumerate(self.content):
                     if i:
                         p.text(",")
                         p.breakable()
                     else:
                         p.breakable("")
                     p.pretty(child)
 
 
 class MyDict(dict):
     def _repr_pretty_(self, p, cycle):
         p.text("MyDict(...)")
 
 
 class Dummy1(object):
     def _repr_pretty_(self, p, cycle):
         p.text("Dummy1(...)")
 
 class Dummy2(Dummy1):
     _repr_pretty_ = None
 
 
 def test_indentation():
     """Test correct indentation in groups"""
     count = 40
     gotoutput = pretty.pretty(MyList(range(count)))
     expectedoutput = "MyList(\n" + ",\n".join("   %d" % i for i in range(count)) + ")"
 
     nt.assert_equal(gotoutput, expectedoutput)
 
 
 def test_dispatch():
     """
     Test correct dispatching: The _repr_pretty_ method for MyDict
     must be found before the registered printer for dict.
     """
     gotoutput = pretty.pretty(MyDict())
     expectedoutput = "MyDict(...)"
 
     nt.assert_equal(gotoutput, expectedoutput)
 
 
 def test_callability_checking():
     """
     Test that the _repr_pretty_ method is tested for callability and skipped if
     not.
     """
     gotoutput = pretty.pretty(Dummy2())
     expectedoutput = "Dummy1(...)"
 
     nt.assert_equal(gotoutput, expectedoutput)
 
+
+def test_sets():
+    """
+    Test that set and frozenset use Python 3 formatting.
+    """
+    objects = [set(), frozenset(), set([1]), frozenset([1]), set([1, 2]),
+        frozenset([1, 2]), set([-1, -2, -3])]
+    expected = ['set()', 'frozenset()', '{1}', 'frozenset({1})', '{1, 2}',
+        'frozenset({1, 2})', '{-3, -2, -1}']
+    for obj, expected_output in zip(objects, expected):
+        got_output = pretty.pretty(obj)
+        yield nt.assert_equal, got_output, expected_output
+
+
 @skip_without('xxlimited')
 def test_pprint_heap_allocated_type():
     """
     Test that pprint works for heap allocated types.
     """
     import xxlimited
     output = pretty.pretty(xxlimited.Null)
     nt.assert_equal(output, 'xxlimited.Null')