shallow and deep copy operations

Lib/copy.py

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