In-Place Operators in Python | Set 1 (iadd (), isub (), iconcat () …)

File handling | iadd | Python Methods and Functions

Python provides methods in its definition to perform operations in place, i.e.  perform assignment and evaluation in one expression using the operator module. For example,

 x + = y is equivalent to x = operator.iadd (x, y) 

Some important in-place operations :

1. iadd () : — this function is used to assign and add the current value . This operation performs the operation " a + = b ". The not assignment is true for immutable containers such as strings, numbers, and tuples.

2. iconcat () : — This function is used to concatenate one line at the end of a second.

# Python code to demonstrate how it works
# iadd () and iconcat ()

# import statement to handle statement operations

import operator

# using iadd () to add and assign a value

x = operator.iadd ( 2 , 3 ); 

# print the changed value

print ( "The value after adding and assigning:" , end = "")

print (x)

# initializing values ​​

y = "geeks"


z = "forgeeks"

# using iconcat () to combine sequences

y = operator.iconcat (y, z)

# using iconcat () to combine sequences

print ( "The string after concatenation is:" , end = "")

print (y)


 The value after adding and assigning: 5 The string after concatenation is: pythonengineering 

3. isub () : — This function is used to assign and subtract the current value . This operation performs the " a- = b " operation. The not assignment is true for immutable containers such as strings, numbers, and tuples.

4. imul () : — This function is used to assign and multiply the current value . This operation performs the operation " a * = b ". The not assignment is true for immutable containers such as strings, numbers, and tuples.

# Python code to demonstrate how it works
# isub () and imul ()

# import statement to handle statement operations

import operator

# using isub () to subtract and assign a value

x = operator.isub ( 2 , 3 ); 

# print the changed value

print ( "The value after subtracting and assigning:" , end = "")

print (x)

# using imul () to multiply and assign a value

x = operator.imul ( 2 , 3 ); 

# print the changed value

print ( "The value after multiplying and assigning:" , end = "")

print (x)


 The value after subtracting and assigning: -1 The value after multiplying and assigning: 6 

5. itruediv () : — This function is used to assign and divide the current value . This operation performs the " a / = b " operation. The not assignment is true for immutable containers such as strings, numbers, and tuples.

6. imod () : — This function is used to assign and return the remainder . This operation performs the operation " a% = b ". The not assignment is true for immutable containers such as strings, numbers, and tuples.

# Python code to demonstrate how it works
# itruediv () and imod ()

# import statement to handle statement operations

import operator

# using itruediv () to divide and assign a value

x = operator.itruediv ( 10 , 5 ); 

# print the changed value

print ( "The value after dividing and assigning:" , end = "")

print (x)

# using imod () for the module and assigning a value

x = operator.imod ( 10 , 6 ); 

# print the changed value

print ( "The value after modulus and assigning:" , end = "")

print (x)


 The value after dividing and assigning: 2.0 The value after modulus and assigning: 4 

Following articles

This article courtesy of Manjit Singh ... If you are as Python.Engineering and would like to contribute, you can also write an article using or by posting an article contribute @ See my article appearing on the Python.Engineering homepage and help other geeks.

Please post comments if you find anything wrong or if you would like to share more information on the topic discussed above.

In-Place Operators in Python | Set 1 (iadd (), isub (), iconcat () ...): StackOverflow Questions

Answer #1

I tested most suggested solutions with perfplot (a pet project of mine, essentially a wrapper around timeit), and found

import functools
import operator
functools.reduce(operator.iconcat, a, [])

to be the fastest solution, both when many small lists and few long lists are concatenated. (operator.iadd is equally fast.)

enter image description here

enter image description here

Code to reproduce the plot:

import functools
import itertools
import numpy
import operator
import perfplot

def forfor(a):
    return [item for sublist in a for item in sublist]

def sum_brackets(a):
    return sum(a, [])

def functools_reduce(a):
    return functools.reduce(operator.concat, a)

def functools_reduce_iconcat(a):
    return functools.reduce(operator.iconcat, a, [])

def itertools_chain(a):
    return list(itertools.chain.from_iterable(a))

def numpy_flat(a):
    return list(numpy.array(a).flat)

def numpy_concatenate(a):
    return list(numpy.concatenate(a))
    setup=lambda n: [list(range(10))] * n,
    # setup=lambda n: [list(range(n))] * 10,
    n_range=[2 ** k for k in range(16)],
    xlabel="num lists (of length 10)",
    # xlabel="len lists (10 lists total)"

Answer #2

What is the difference between the list methods append and extend?

  • append adds its argument as a single element to the end of a list. The length of the list itself will increase by one.
  • extend iterates over its argument adding each element to the list, extending the list. The length of the list will increase by however many elements were in the iterable argument.


The list.append method appends an object to the end of the list.


Whatever the object is, whether a number, a string, another list, or something else, it gets added onto the end of my_list as a single entry on the list.

>>> my_list
["foo", "bar"]
>>> my_list.append("baz")
>>> my_list
["foo", "bar", "baz"]

So keep in mind that a list is an object. If you append another list onto a list, the first list will be a single object at the end of the list (which may not be what you want):

>>> another_list = [1, 2, 3]
>>> my_list.append(another_list)
>>> my_list
["foo", "bar", "baz", [1, 2, 3]]
                     #^^^^^^^^^--- single item at the end of the list.


The list.extend method extends a list by appending elements from an iterable:


So with extend, each element of the iterable gets appended onto the list. For example:

>>> my_list
["foo", "bar"]
>>> another_list = [1, 2, 3]
>>> my_list.extend(another_list)
>>> my_list
["foo", "bar", 1, 2, 3]

Keep in mind that a string is an iterable, so if you extend a list with a string, you"ll append each character as you iterate over the string (which may not be what you want):

>>> my_list.extend("baz")
>>> my_list
["foo", "bar", 1, 2, 3, "b", "a", "z"]

Operator Overload, __add__ (+) and __iadd__ (+=)

Both + and += operators are defined for list. They are semantically similar to extend.

my_list + another_list creates a third list in memory, so you can return the result of it, but it requires that the second iterable be a list.

my_list += another_list modifies the list in-place (it is the in-place operator, and lists are mutable objects, as we"ve seen) so it does not create a new list. It also works like extend, in that the second iterable can be any kind of iterable.

Don"t get confused - my_list = my_list + another_list is not equivalent to += - it gives you a brand new list assigned to my_list.

Time Complexity

Append has (amortized) constant time complexity, O(1).

Extend has time complexity, O(k).

Iterating through the multiple calls to append adds to the complexity, making it equivalent to that of extend, and since extend"s iteration is implemented in C, it will always be faster if you intend to append successive items from an iterable onto a list.

Regarding "amortized" - from the list object implementation source:

    /* This over-allocates proportional to the list size, making room
     * for additional growth.  The over-allocation is mild, but is
     * enough to give linear-time amortized behavior over a long
     * sequence of appends() in the presence of a poorly-performing
     * system realloc().

This means that we get the benefits of a larger than needed memory reallocation up front, but we may pay for it on the next marginal reallocation with an even larger one. Total time for all appends is linear at O(n), and that time allocated per append, becomes O(1).


You may wonder what is more performant, since append can be used to achieve the same outcome as extend. The following functions do the same thing:

def append(alist, iterable):
    for item in iterable:
def extend(alist, iterable):

So let"s time them:

import timeit

>>> min(timeit.repeat(lambda: append([], "abcdefghijklmnopqrstuvwxyz")))
>>> min(timeit.repeat(lambda: extend([], "abcdefghijklmnopqrstuvwxyz")))

Addressing a comment on timings

A commenter said:

Perfect answer, I just miss the timing of comparing adding only one element

Do the semantically correct thing. If you want to append all elements in an iterable, use extend. If you"re just adding one element, use append.

Ok, so let"s create an experiment to see how this works out in time:

def append_one(a_list, element):

def extend_one(a_list, element):
    """creating a new list is semantically the most direct
    way to create an iterable to give to extend"""

import timeit

And we see that going out of our way to create an iterable just to use extend is a (minor) waste of time:

>>> min(timeit.repeat(lambda: append_one([], 0)))
>>> min(timeit.repeat(lambda: extend_one([], 0)))

We learn from this that there"s nothing gained from using extend when we have only one element to append.

Also, these timings are not that important. I am just showing them to make the point that, in Python, doing the semantically correct thing is doing things the Right Way‚Ñ¢.

It"s conceivable that you might test timings on two comparable operations and get an ambiguous or inverse result. Just focus on doing the semantically correct thing.


We see that extend is semantically clearer, and that it can run much faster than append, when you intend to append each element in an iterable to a list.

If you only have a single element (not in an iterable) to add to the list, use append.

Answer #3

How do I concatenate two lists in Python?

As of 3.9, these are the most popular stdlib methods for concatenating two (or more) lists in python.

enter image description here


  1. This is a slick solution because of its succinctness. But sum performs concatenation in a pairwise fashion, which means this is a quadratic operation as memory has to be allocated for each step. DO NOT USE if your lists are large.

  2. See chain and chain.from_iterable from the docs. You will need to import itertools first. Concatenation is linear in memory, so this is the best in terms of performance and version compatibility. chain.from_iterable was introduced in 2.6.

  3. This method uses Additional Unpacking Generalizations (PEP 448), but cannot generalize to N lists unless you manually unpack each one yourself.

  4. a += b and a.extend(b) are more or less equivalent for all practical purposes. += when called on a list will internally call list.__iadd__, which extends the first list by the second.


2-List Concatenation1

enter image description here

There"s not much difference between these methods but that makes sense given they all have the same order of complexity (linear). There"s no particular reason to prefer one over the other except as a matter of style.

N-List Concatenation

enter image description here

Plots have been generated using the perfplot module. Code, for your reference.

1. The iadd (+=) and extend methods operate in-place, so a copy has to be generated each time before testing. To keep things fair, all methods have a pre-copy step for the left-hand list which can be ignored.

Comments on Other Solutions

  • DO NOT USE THE DUNDER METHOD list.__add__ directly in any way, shape or form. In fact, stay clear of dunder methods, and use the operators and operator functions like they were designed for. Python has careful semantics baked into these which are more complicated than just calling the dunder directly. Here is an example. So, to summarise, a.__add__(b) => BAD; a + b => GOOD.

  • Some answers here offer reduce(operator.add, [a, b]) for pairwise concatenation -- this is the same as sum([a, b], []) only more wordy.

  • Any method that uses set will drop duplicates and lose ordering. Use with caution.

  • for i in b: a.append(i) is more wordy, and slower than a.extend(b), which is single function call and more idiomatic. append is slower because of the semantics with which memory is allocated and grown for lists. See here for a similar discussion.

  • heapq.merge will work, but its use case is for merging sorted lists in linear time. Using it in any other situation is an anti-pattern.

  • yielding list elements from a function is an acceptable method, but chain does this faster and better (it has a code path in C, so it is fast).

  • operator.add(a, b) is an acceptable functional equivalent to a + b. It"s use cases are mainly for dynamic method dispatch. Otherwise, prefer a + b which is shorter and more readable, in my opinion. YMMV.

Answer #4

The difference is that one modifies the data-structure itself (in-place operation) b += 1 while the other just reassigns the variable a = a + 1.

Just for completeness:

x += y is not always doing an in-place operation, there are (at least) three exceptions:

  • If x doesn"t implement an __iadd__ method then the x += y statement is just a shorthand for x = x + y. This would be the case if x was something like an int.

  • If __iadd__ returns NotImplemented, Python falls back to x = x + y.

  • The __iadd__ method could theoretically be implemented to not work in place. It"d be really weird to do that, though.

As it happens your bs are numpy.ndarrays which implements __iadd__ and return itself so your second loop modifies the original array in-place.

You can read more on this in the Python documentation of "Emulating Numeric Types".

These [__i*__] methods are called to implement the augmented arithmetic assignments (+=, -=, *=, @=, /=, //=, %=, **=, <<=, >>=, &=, ^=, |=). These methods should attempt to do the operation in-place (modifying self) and return the result (which could be, but does not have to be, self). If a specific method is not defined, the augmented assignment falls back to the normal methods. For instance, if x is an instance of a class with an __iadd__() method, x += y is equivalent to x = x.__iadd__(y) . Otherwise, x.__add__(y) and y.__radd__(x) are considered, as with the evaluation of x + y. In certain situations, augmented assignment can result in unexpected errors (see Why does a_tuple[i] += ["item"] raise an exception when the addition works?), but this behavior is in fact part of the data model.

Answer #5

This depends entirely on the object i.

+= calls the __iadd__ method (if it exists -- falling back on __add__ if it doesn"t exist) whereas + calls the __add__ method1 or the __radd__ method in a few cases2.

From an API perspective, __iadd__ is supposed to be used for modifying mutable objects in place (returning the object which was mutated) whereas __add__ should return a new instance of something. For immutable objects, both methods return a new instance, but __iadd__ will put the new instance in the current namespace with the same name that the old instance had. This is why

i = 1
i += 1

seems to increment i. In reality, you get a new integer and assign it "on top of" i -- losing one reference to the old integer. In this case, i += 1 is exactly the same as i = i + 1. But, with most mutable objects, it"s a different story:

As a concrete example:

a = [1, 2, 3]
b = a
b += [1, 2, 3]
print a  #[1, 2, 3, 1, 2, 3]
print b  #[1, 2, 3, 1, 2, 3]

compared to:

a = [1, 2, 3]
b = a
b = b + [1, 2, 3]
print a #[1, 2, 3]
print b #[1, 2, 3, 1, 2, 3]

notice how in the first example, since b and a reference the same object, when I use += on b, it actually changes b (and a sees that change too -- After all, it"s referencing the same list). In the second case however, when I do b = b + [1, 2, 3], this takes the list that b is referencing and concatenates it with a new list [1, 2, 3]. It then stores the concatenated list in the current namespace as b -- With no regard for what b was the line before.

1In the expression x + y, if x.__add__ isn"t implemented or if x.__add__(y) returns NotImplemented and x and y have different types, then x + y tries to call y.__radd__(x). So, in the case where you have

foo_instance += bar_instance

if Foo doesn"t implement __add__ or __iadd__ then the result here is the same as

foo_instance = bar_instance.__radd__(bar_instance, foo_instance)

2In the expression foo_instance + bar_instance, bar_instance.__radd__ will be tried before foo_instance.__add__ if the type of bar_instance is a subclass of the type of foo_instance (e.g. issubclass(Bar, Foo)). The rationale for this is that Bar is in some sense a "higher-level" object than Foo so Bar should get the option of overriding Foo"s behavior.

Answer #6

From the documentation:

The @ (at) operator is intended to be used for matrix multiplication. No builtin Python types implement this operator.

The @ operator was introduced in Python 3.5. @= is matrix multiplication followed by assignment, as you would expect. They map to __matmul__, __rmatmul__ or __imatmul__ similar to how + and += map to __add__, __radd__ or __iadd__.

The operator and the rationale behind it are discussed in detail in PEP 465.

Answer #7

In Python, += is sugar coating for the __iadd__ special method, or __add__ or __radd__ if __iadd__ isn"t present. The __iadd__ method of a class can do anything it wants. The list object implements it and uses it to iterate over an iterable object appending each element to itself in the same way that the list"s extend method does.

Here"s a simple custom class that implements the __iadd__ special method. You initialize the object with an int, then can use the += operator to add a number. I"ve added a print statement in __iadd__ to show that it gets called. Also, __iadd__ is expected to return an object, so I returned the addition of itself plus the other number which makes sense in this case.

>>> class Adder(object):
        def __init__(self, num=0):
            self.num = num

        def __iadd__(self, other):
            print "in __iadd__", other
            self.num = self.num + other
            return self.num

>>> a = Adder(2)
>>> a += 3
in __iadd__ 3
>>> a

Hope this helps.

Answer #8

The general answer is that += tries to call the __iadd__ special method, and if that isn"t available it tries to use __add__ instead. So the issue is with the difference between these special methods.

The __iadd__ special method is for an in-place addition, that is it mutates the object that it acts on. The __add__ special method returns a new object and is also used for the standard + operator.

So when the += operator is used on an object which has an __iadd__ defined the object is modified in place. Otherwise it will instead try to use the plain __add__ and return a new object.

That is why for mutable types like lists += changes the object"s value, whereas for immutable types like tuples, strings and integers a new object is returned instead (a += b becomes equivalent to a = a + b).

For types that support both __iadd__ and __add__ you therefore have to be careful which one you use. a += b will call __iadd__ and mutate a, whereas a = a + b will create a new object and assign it to a. They are not the same operation!

>>> a1 = a2 = [1, 2]
>>> b1 = b2 = [1, 2]
>>> a1 += [3]          # Uses __iadd__, modifies a1 in-place
>>> b1 = b1 + [3]      # Uses __add__, creates new list, assigns it to b1
>>> a2
[1, 2, 3]              # a1 and a2 are still the same list
>>> b2
[1, 2]                 # whereas only b1 was changed

For immutable types (where you don"t have an __iadd__) a += b and a = a + b are equivalent. This is what lets you use += on immutable types, which might seem a strange design decision until you consider that otherwise you couldn"t use += on immutable types like numbers!