How to clone or copy a list?
What are the options to clone or copy a list in Python?
Using new_list = my_list
then modifies new_list
every time my_list
changes.
Why is this?
With new_list = my_list
, you don't actually have two lists. The assignment just copies the reference to the list, not the actual list, so both new_list
and my_list
refer to the same list after the assignment.
To actually copy the list, you have various possibilities:
You can slice it:
new_list = old_list[:]
Alex Martelli's opinion (at least back in 2007) about this is, that it is a weird syntax and it does not make sense to use it ever. ;) (In his opinion, the next one is more readable).
You can use the built in list()
function:
new_list = list(old_list)
You can use generic copy.copy()
:
import copy
new_list = copy.copy(old_list)
This is a little slower than list()
because it has to find out the datatype of old_list
first.
If the list contains objects and you want to copy them as well, use generic copy.deepcopy()
:
import copy
new_list = copy.deepcopy(old_list)
Obviously the slowest and most memory-needing method, but sometimes unavoidable.
Example:
import copy
class Foo(object):
def __init__(self, val):
self.val = val
def __repr__(self):
return str(self.val)
foo = Foo(1)
a = ['foo', foo]
b = a[:]
c = list(a)
d = copy.copy(a)
e = copy.deepcopy(a)
# edit orignal list and instance
a.append('baz')
foo.val = 5
print('original: %rn slice: %rn list(): %rn copy: %rn deepcopy: %r'
% (a, b, c, d, e))
Result:
original: ['foo', 5, 'baz']
slice: ['foo', 5]
list(): ['foo', 5]
copy: ['foo', 5]
deepcopy: ['foo', 1]
Felix already provided an excellent answer, but I thought I'd do a speed comparison of the various methods:
copy.deepcopy(old_list)
Copy()
method copying classes with deepcopy Copy()
method not copying classes (only dicts/lists/tuples) for item in old_list: new_list.append(item)
[i for i in old_list]
(a list comprehension) copy.copy(old_list)
list(old_list)
new_list = []; new_list.extend(old_list)
new_list = []; new_list.extend(old_list)
old_list[:]
(list slicing) So the fastest is list slicing. But be aware that copy.copy()
, list[:]
and list(list)
, unlike copy.deepcopy()
and the python version don't copy any lists, dictionaries and class instances in the list, so if the originals change, they will change in the copied list too and vice versa.
(Here's the script if anyone's interested or wants to raise any issues:)
from copy import deepcopy
class old_class:
def __init__(self):
self.blah = 'blah'
class new_class(object):
def __init__(self):
self.blah = 'blah'
dignore = {str: None, unicode: None, int: None, type(None): None}
def Copy(obj, use_deepcopy=True):
t = type(obj)
if t in (list, tuple):
if t == tuple:
# Convert to a list if a tuple to
# allow assigning to when copying
is_tuple = True
obj = list(obj)
else:
# Otherwise just do a quick slice copy
obj = obj[:]
is_tuple = False
# Copy each item recursively
for x in xrange(len(obj)):
if type(obj[x]) in dignore:
continue
obj[x] = Copy(obj[x], use_deepcopy)
if is_tuple:
# Convert back into a tuple again
obj = tuple(obj)
elif t == dict:
# Use the fast shallow dict copy() method and copy any
# values which aren't immutable (like lists, dicts etc)
obj = obj.copy()
for k in obj:
if type(obj[k]) in dignore:
continue
obj[k] = Copy(obj[k], use_deepcopy)
elif t in dignore:
# Numeric or string/unicode?
# It's immutable, so ignore it!
pass
elif use_deepcopy:
obj = deepcopy(obj)
return obj
if __name__ == '__main__':
import copy
from time import time
num_times = 100000
L = [None, 'blah', 1, 543.4532,
['foo'], ('bar',), {'blah': 'blah'},
old_class(), new_class()]
t = time()
for i in xrange(num_times):
Copy(L)
print 'Custom Copy:', time()-t
t = time()
for i in xrange(num_times):
Copy(L, use_deepcopy=False)
print 'Custom Copy Only Copying Lists/Tuples/Dicts (no classes):', time()-t
t = time()
for i in xrange(num_times):
copy.copy(L)
print 'copy.copy:', time()-t
t = time()
for i in xrange(num_times):
copy.deepcopy(L)
print 'copy.deepcopy:', time()-t
t = time()
for i in xrange(num_times):
L[:]
print 'list slicing [:]:', time()-t
t = time()
for i in xrange(num_times):
list(L)
print 'list(L):', time()-t
t = time()
for i in xrange(num_times):
[i for i in L]
print 'list expression(L):', time()-t
t = time()
for i in xrange(num_times):
a = []
a.extend(L)
print 'list extend:', time()-t
t = time()
for i in xrange(num_times):
a = []
for y in L:
a.append(y)
print 'list append:', time()-t
t = time()
for i in xrange(num_times):
a = []
a.extend(i for i in L)
print 'generator expression extend:', time()-t
EDIT : Added new-style, old-style classes and dicts to the benchmarks, and made the python version much faster and added some more methods including list expressions and extend()
.
我听说Python 3.3+增加了list.copy()
方法,它应该像切片一样快:
newlist = old_list.copy()
上一篇: 从GitHub删除分叉的回购
下一篇: 如何克隆或复制列表?