# Finding and replacing elements in a list

## Question:

I have to search through a list and replace all occurrences of one element with another. So far my attempts in code are getting me nowhere, what is the best way to do this?

For example, suppose my list has the following integers

``````>>> a = [1,2,3,4,5,1,2,3,4,5,1]
``````

and I need to replace all occurrences of the number 1 with the value 10 so the output I need is

``````>>> a = [10, 2, 3, 4, 5, 10, 2, 3, 4, 5, 10]
``````

Thus my goal is to replace all instances of the number 1 with the number 10.

Try using a list comprehension and a conditional expression.

``````>>> a=[1,2,3,1,3,2,1,1]
>>> [4 if x==1 else x for x in a]
[4, 2, 3, 4, 3, 2, 4, 4]
``````
``````>>> a=[1,2,3,4,5,1,2,3,4,5,1]
>>> item_to_replace = 1
>>> replacement_value = 6
>>> indices_to_replace = [i for i,x in enumerate(a) if x==item_to_replace]
>>> indices_to_replace
[0, 5, 10]
>>> for i in indices_to_replace:
...     a[i] = replacement_value
...
>>> a
[6, 2, 3, 4, 5, 6, 2, 3, 4, 5, 6]
>>>
``````

You can use the built-in `enumerate` to get both index and value while iterating the list. Then, use the value to test for a condition and the index to replace that value in the original list:

``````>>> a = [1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 1]
>>> for i, n in enumerate(a):
...   if n == 1:
...      a[i] = 10
...
>>> a
[10, 2, 3, 4, 5, 10, 2, 3, 4, 5, 10]
``````

List comprehension works well, and looping through with enumerate can save you some memory (b/c the operation’s essentially being done in place).

There’s also functional programming. See usage of map:

``````>>> a = [1,2,3,2,3,4,3,5,6,6,5,4,5,4,3,4,3,2,1]
>>> map(lambda x: x if x != 4 else 'sss', a)
[1, 2, 3, 2, 3, 'sss', 3, 5, 6, 6, 5, 'sss', 5, 'sss', 3, 'sss', 3, 2, 1]
``````

If you have several values to replace, you can also use a dictionary:

``````a = [1, 2, 3, 4, 1, 5, 3, 2, 6, 1, 1]
replacements = {1:10, 2:20, 3:'foo'}
replacer = replacements.get  # For faster gets.

print([replacer(n, n) for n in a])

> [10, 20, 'foo', 4, 10, 5, 'foo', 20, 6, 10, 10]
``````

Note that this approach works only if the elements to be replaced are hashable. This is because dict keys are required to be hashable.

I know this is a very old question and there’s a myriad of ways to do it. The simpler one I found is using `numpy` package.

``````import numpy

arr = numpy.asarray([1, 6, 1, 9, 8])
arr[ arr == 8 ] = 0 # change all occurrences of 8 by 0
print(arr)
``````

On long lists and rare occurrences its about 3x faster using `list.index()` – compared to single step iteration methods presented in the other answers.

``````def list_replace(lst, old=1, new=10):
"""replace list elements (inplace)"""
i = -1
try:
while True:
i = lst.index(old, i + 1)
lst[i] = new
except ValueError:
pass
``````

My usecase was replacing `None` with some default value.

I’ve timed approaches to this problem that were presented here, including the one by @kxr – using `str.count`.

Test code in ipython with Python 3.8.1:

``````def rep1(lst, replacer = 0):
''' List comprehension, new list '''

return [item if item is not None else replacer for item in lst]

def rep2(lst, replacer = 0):
''' List comprehension, in-place '''
lst[:] =  [item if item is not None else replacer for item in lst]

return lst

def rep3(lst, replacer = 0):
''' enumerate() with comparison - in-place '''
for idx, item in enumerate(lst):
if item is None:
lst[idx] = replacer

return lst

def rep4(lst, replacer = 0):
''' Using str.index + Exception, in-place '''

idx = -1
# none_amount = lst.count(None)
while True:
try:
idx = lst.index(None, idx+1)
except ValueError:
break
else:
lst[idx] = replacer

return lst

def rep5(lst, replacer = 0):
''' Using str.index + str.count, in-place '''

idx = -1
for _ in range(lst.count(None)):
idx = lst.index(None, idx+1)
lst[idx] = replacer

return lst

def rep6(lst, replacer = 0):
''' Using map, return map iterator '''

return map(lambda item: item if item is not None else replacer, lst)

def rep7(lst, replacer = 0):
''' Using map, return new list '''

return list(map(lambda item: item if item is not None else replacer, lst))

lst = [5]*10**6
# lst = [None]*10**6

%timeit rep1(lst)
%timeit rep2(lst)
%timeit rep3(lst)
%timeit rep4(lst)
%timeit rep5(lst)
%timeit rep6(lst)
%timeit rep7(lst)
``````

I get:

``````26.3 ms ± 163 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
29.3 ms ± 206 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
33.8 ms ± 191 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
11.9 ms ± 37.8 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
11.9 ms ± 60.2 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
260 ns ± 1.84 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
56.5 ms ± 204 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
``````

Using the internal `str.index` is in fact faster than any manual comparison.

I didn’t know if the exception in test 4 would be more laborious than using `str.count`, the difference seems negligible.

Note that `map()` (test 6) returns an iterator and not an actual list, thus test 7.

The answers for this old but relevant question are wildly variable in speed.

The fastest of the solution posted by kxr.

However, this is even faster and otherwise not here:

``````def f1(arr, find, replace):
base=0
for cnt in range(arr.count(find)):
offset=arr.index(find, base)
arr[offset]=replace
base=offset+1
``````

Here is timing for the various solutions. The faster ones are 3X faster than accepted answer and 5X faster than the slowest answer here.

To be fair, all methods needed to do inlace replacement of the array sent to the function.

``````def f1(arr, find, replace):
base=0
for cnt in range(arr.count(find)):
offset=arr.index(find, base)
arr[offset]=replace
base=offset+1

def f2(arr,find,replace):
for i,e in enumerate(arr):
if e==find:
arr[i]=replace

def f3(arr,find,replace):
# in place list comprehension
arr[:]=[replace if e==find else e for e in arr]

def f4(arr,find,replace):
# in place map and lambda -- SLOW
arr[:]=list(map(lambda x: x if x != find else replace, arr))

def f5(arr,find,replace):
# find index with comprehension
for i in [i for i, e in enumerate(arr) if e==find]:
arr[i]=replace

def f6(arr,find,replace):
# FASTEST but a little les clear
try:
while True:
arr[arr.index(find)]=replace
except ValueError:
pass

def f7(lst, old, new):
"""replace list elements (inplace)"""
i = -1
try:
while 1:
i = lst.index(old, i + 1)
lst[i] = new
except ValueError:
pass

import time

def cmpthese(funcs, args=(), cnt=1000, rate=True, micro=True):
"""Generate a Perl style function benchmark"""
def pprint_table(table):
"""Perl style table output"""
def format_field(field, fmt='{:,.0f}'):
if type(field) is str: return field
if type(field) is tuple: return field[1].format(field[0])
return fmt.format(field)

def get_max_col_w(table, index):
return max([len(format_field(row[index])) for row in table])

col_paddings=[get_max_col_w(table, i) for i in range(len(table[0]))]
for i,row in enumerate(table):
# left col
# rest of the cols
print(' '.join(row_tab))

results={}
for i in range(cnt):
for f in funcs:
start=time.perf_counter_ns()
f(*args)
stop=time.perf_counter_ns()
results.setdefault(f.__name__, []).append(stop-start)
results={k:float(sum(v))/len(v) for k,v in results.items()}
fastest=sorted(results,key=results.get, reverse=True)
table=[['']]
if rate: table[0].append('rate/sec')
if micro: table[0].append('u03bcsec/pass')
table[0].extend(fastest)
for e in fastest:
tmp=[e]
if rate:
tmp.append('{:,}'.format(int(round(float(cnt)*1000000.0/results[e]))))

if micro:
tmp.append('{:,.1f}'.format(results[e]/float(cnt)))

for x in fastest:
if x==e: tmp.append('--')
else: tmp.append('{:.1%}'.format((results[x]-results[e])/results[e]))
table.append(tmp)

pprint_table(table)

if __name__=='__main__':
import sys
import time
print(sys.version)
cases=(
('small, found', 9, 100),
('large, found', 9, 1000),
)
for txt, tgt, mul in cases:
print(f'n{txt}:')
arr=[1,2,3,4,5,6,7,8,9,0]*mul
args=(arr,tgt,'X')
cmpthese([f1,f2,f3, f4, f5, f6, f7],args)
``````

And the results:

``````3.9.1 (default, Feb  3 2021, 07:38:02)
[Clang 12.0.0 (clang-1200.0.32.29)]

small, found:
rate/sec μsec/pass     f4     f3     f5     f2     f6     f7     f1
f4  133,982       7.5     -- -38.8% -49.0% -52.5% -78.5% -78.6% -82.9%
f3  219,090       4.6  63.5%     -- -16.6% -22.4% -64.8% -65.0% -72.0%
f5  262,801       3.8  96.1%  20.0%     --  -6.9% -57.8% -58.0% -66.4%
f2  282,259       3.5 110.7%  28.8%   7.4%     -- -54.6% -54.9% -63.9%
f6  622,122       1.6 364.3% 184.0% 136.7% 120.4%     --  -0.7% -20.5%
f7  626,367       1.6 367.5% 185.9% 138.3% 121.9%   0.7%     -- -19.9%
f1  782,307       1.3 483.9% 257.1% 197.7% 177.2%  25.7%  24.9%     --

rate/sec μsec/pass     f4     f5     f2     f3     f6     f7     f1
f4   13,846      72.2     -- -40.3% -41.4% -47.8% -85.2% -85.4% -86.2%
f5   23,186      43.1  67.5%     --  -1.9% -12.5% -75.2% -75.5% -76.9%
f2   23,646      42.3  70.8%   2.0%     -- -10.8% -74.8% -75.0% -76.4%
f3   26,512      37.7  91.5%  14.3%  12.1%     -- -71.7% -72.0% -73.5%
f6   93,656      10.7 576.4% 303.9% 296.1% 253.3%     --  -1.0%  -6.5%
f7   94,594      10.6 583.2% 308.0% 300.0% 256.8%   1.0%     --  -5.6%
f1  100,206      10.0 623.7% 332.2% 323.8% 278.0%   7.0%   5.9%     --

large, found:
rate/sec μsec/pass     f4     f2     f5     f3     f6     f7     f1
f4      145   6,889.4     -- -33.3% -34.8% -48.6% -85.3% -85.4% -85.8%
f2      218   4,593.5  50.0%     --  -2.2% -22.8% -78.0% -78.1% -78.6%
f5      223   4,492.4  53.4%   2.3%     -- -21.1% -77.5% -77.6% -78.2%
f3      282   3,544.0  94.4%  29.6%  26.8%     -- -71.5% -71.6% -72.3%
f6      991   1,009.5 582.4% 355.0% 345.0% 251.1%     --  -0.4%  -2.8%
f7      995   1,005.4 585.2% 356.9% 346.8% 252.5%   0.4%     --  -2.4%
f1    1,019     981.3 602.1% 368.1% 357.8% 261.2%   2.9%   2.5%     --

rate/sec μsec/pass     f4     f5     f2     f3     f6     f7     f1
f4      147   6,812.0     -- -35.0% -36.4% -48.9% -85.7% -85.8% -86.1%
f5      226   4,424.8  54.0%     --  -2.0% -21.3% -78.0% -78.1% -78.6%
f2      231   4,334.9  57.1%   2.1%     -- -19.6% -77.6% -77.7% -78.2%
f3      287   3,484.0  95.5%  27.0%  24.4%     -- -72.1% -72.2% -72.8%
f6    1,028     972.3 600.6% 355.1% 345.8% 258.3%     --  -0.4%  -2.7%
f7    1,033     968.2 603.6% 357.0% 347.7% 259.8%   0.4%     --  -2.3%
f1    1,057     946.2 619.9% 367.6% 358.1% 268.2%   2.8%   2.3%     --
``````

I might be a dumb-dumb, but I would write a separate, simple function for this:

``````def convertElements( oldlist, convert_dict ):
newlist = []
for e in oldlist:
if e in convert_dict:
newlist.append(convert_dict[e])
else:
newlist.append(e)
return newlist
``````

And then call this as needed like so:

``````a = [1,2,3,4,5,1,2,3,4,5,1]
a_new = convertElements(a, {1: 10})
## OUTPUT: a_new=[10, 2, 3, 4, 5, 10, 2, 3, 4, 5, 10]
``````
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