shallow and deep copy operations
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Guido van Rossum
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# Generic (shallow and deep) copying operations
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# =============================================
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#
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# The difference between shallow and deep copying is only relevant for
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# compound objects (objects that contain other objects, like lists or class
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# instances).
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#
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# - A shallow copy constructs a new compound object and then (to the extent
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# possible) inserts *the same objects* into in that the original contains.
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#
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# - A deep copy constructs a new compound object and then, recursively,
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# inserts *copies* into it of the objects found in the original.
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#
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# Two problems often exist with deep copy operations that don't exist with
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# shallow copy operations:
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#
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# (a) recursive objects (compound objects that, directly or indirectly,
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# contain a reference to themselves) may cause a recursive loop
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#
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# (b) because deep copy copies *everything* it may copy too much, e.g.
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# administrative data structures that should be shared even between copies
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#
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# Python's deep copy operation avoids these problems by:
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#
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# (a) keeping a table of objects already copied during the current copying pass
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#
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# (b) letting user-defined classes override the copying operation or the set
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# of components copied
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#
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# This version does not copy types like module, class, function, method,
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# nor stack trace, stack frame, nor file, socket, window, nor array,
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# nor any similar types.
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import types
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Error = 'copy.Error'
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def copy(x):
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try:
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copierfunction = _copy_dispatch[type(x)]
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except KeyError:
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try:
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copier = x.__copy__
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except AttributeError:
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raise Error, "un(shallow)copyable object of type %s" % type(x)
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y = copier()
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else:
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y = copierfunction(x)
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return y
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_copy_dispatch = d = {}
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def _copy_atomic(x):
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return x
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d[types.NoneType] = _copy_atomic
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d[types.IntType] = _copy_atomic
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d[types.LongType] = _copy_atomic
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d[types.FloatType] = _copy_atomic
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d[types.StringType] = _copy_atomic
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d[types.CodeType] = _copy_atomic
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d[types.TypeType] = _copy_atomic
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d[types.XRangeType] = _copy_atomic
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def _copy_list(x):
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return x[:]
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d[types.ListType] = _copy_list
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def _copy_tuple(x):
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return x[:]
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d[types.TupleType] = _copy_tuple
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def _copy_dict(x):
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y = {}
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for key in x.keys():
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y[key] = x[key]
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return y
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d[types.DictionaryType] = _copy_dict
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def _copy_inst(x):
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if hasattr(x, '__copy__'):
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return x.__copy__()
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if hasattr(x, '__getinitargs__'):
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args = x.__getinitargs__()
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else:
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args = ()
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y = apply(x.__class__, args)
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if hasattr(x, '__getstate__'):
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state = x.__getstate__()
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else:
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state = x.__dict__
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if y.hasattr('__setstate__'):
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y.__setstate__(state)
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else:
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for key in state.keys():
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setattr(y, key, state[key])
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return y
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d[types.InstanceType] = _copy_inst
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del d
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def deepcopy(x, memo = None):
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if memo is None:
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memo = {}
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d = id(x)
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if memo.has_key(d):
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return memo[d]
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try:
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copierfunction = _deepcopy_dispatch[type(x)]
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except KeyError:
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try:
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copier = x.__deepcopy__
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except AttributeError:
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raise Error, "un-deep-copyable object of type %s" % type(x)
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y = copier(memo)
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else:
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y = copierfunction(x, memo)
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memo[d] = y
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return y
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_deepcopy_dispatch = d = {}
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def _deepcopy_atomic(x, memo):
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return x
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d[types.NoneType] = _deepcopy_atomic
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d[types.IntType] = _deepcopy_atomic
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d[types.LongType] = _deepcopy_atomic
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d[types.FloatType] = _deepcopy_atomic
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d[types.StringType] = _deepcopy_atomic
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d[types.CodeType] = _deepcopy_atomic
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d[types.TypeType] = _deepcopy_atomic
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d[types.XRangeType] = _deepcopy_atomic
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def _deepcopy_list(x, memo):
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y = []
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memo[id(x)] = y
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for a in x:
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y.append(deepcopy(a, memo))
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return y
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d[types.ListType] = _deepcopy_list
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def _deepcopy_tuple(x, memo):
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y = []
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for a in x:
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y.append(deepcopy(a, memo))
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d = id(x)
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try:
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return memo[d]
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except KeyError:
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pass
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for i in range(len(x)):
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if x[i] is not y[i]:
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return tuple(y)
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return x
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d[types.TupleType] = _deepcopy_tuple
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def _deepcopy_dict(x, memo):
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y = {}
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memo[id(x)] = y
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for key in x.keys():
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y[deepcopy(key, memo)] = deepcopy(x[key], memo)
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return y
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d[types.DictionaryType] = _deepcopy_dict
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def _deepcopy_inst(x, memo):
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if hasattr(x, '__deepcopy__'):
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return x.__deepcopy__()
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if hasattr(x, '__getinitargs__'):
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args = x.__getinitargs__()
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args = deepcopy(args, memo)
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else:
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args = ()
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y = apply(x.__class__, args)
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memo[id(x)] = y
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if hasattr(x, '__getstate__'):
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state = x.__getstate__()
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else:
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state = x.__dict__
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state = deepcopy(state, memo)
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if y.hasattr('__setstate__'):
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y.__setstate__(state)
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else:
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for key in state.keys():
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setattr(y, key, state[key])
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return y
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d[types.InstanceType] = _deepcopy_inst
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del d
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del types
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def _test():
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l = [None, 1, 2L, 3.14, 'xyzzy', (1, 2L), [3.14, 'abc'], {'abc': 'ABC'}, (), [], {}]
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l1 = copy(l)
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print l1==l
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l1 = map(copy, l)
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print l1==l
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l1 = deepcopy(l)
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print l1==l
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class C:
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def __init__(self, arg=None):
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self.a = 1
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self.arg = arg
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self.fp = open('copy.py')
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self.fp.close()
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def __getstate__(self):
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return {'a': self.a, 'arg': self.arg}
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def __setstate__(self, state):
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for key in state.keys():
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setattr(self, key, state[key])
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def __deepcopy__(self, memo = None):
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new = self.__class__(deepcopy(self.arg, memo))
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new.a = self.a
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return new
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c = C('argument sketch')
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l.append(c)
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l2 = copy(l)
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print l == l2
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print l
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print l2
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l2 = deepcopy(l)
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print l == l2
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print l
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print l2
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l.append({l[1]: l, 'xyz': l[2]})
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l3 = copy(l)
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import repr
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print map(repr.repr, l)
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print map(repr.repr, l1)
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print map(repr.repr, l2)
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print map(repr.repr, l3)
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l3 = deepcopy(l)
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import repr
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print map(repr.repr, l)
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print map(repr.repr, l1)
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print map(repr.repr, l2)
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print map(repr.repr, l3)
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if __name__ == '__main__':
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_test()
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