This question already has answers here:
How do I clone a list so that it doesn't change unexpectedly after assignment?
(24 answers)
Is i = i + n truly the same as i += n? [duplicate]
(3 answers)
Closed 2 years ago.
So this is happening:
a = [1,2]
b = a
b += [3]
print(a)
[1, 2, 3]
It's extremely confusing and it's screwing up my project's test suite, which takes a list of strings and tests that it gets modified in the correct ways by each function.
Did I just find a bug in Python, or is there some convoluted reason for this misbehavior?
Related
This question already has answers here:
How do I clone a list so that it doesn't change unexpectedly after assignment?
(24 answers)
Closed last month.
a = [1,2,4]
b=a
b.insert(1,45)
print(a,b)
results
a = [1,45,2,4]
b = [1,45,2,4]
why a is changing
is there any way, where b will change
That's because you're just reassigning the same instance. You need to do this instead of b=a:
b = a.copy()
This question already has answers here:
List of lists changes reflected across sublists unexpectedly
(17 answers)
Closed 4 years ago.
image=[[1,1,1],[1,1,0],[1,0,1]]
visited = [[False] * len(image[0])] * len(image)
visited[0][0] = True
print((visited[1][0]))
Shouldn't the above python code print False? Why is the entire column being assigned as True?
The reference to [False] * len(image [0]) is repeated len(image) times, so modifying the first element of visited affects the second (and third).
This question already has answers here:
Understanding the "is" operator [duplicate]
(11 answers)
Two variables in Python have same id, but not lists or tuples
(5 answers)
Closed 5 years ago.
I understand that the "is" operator checks identity between two objects, but what is this?
a=25
b=25
a is b
True
Why is this true if
a = [1,2,3]
b = [1,2,3]
a is b
False
And from this post, https://www.quora.com/Python-is-operator, it says all immutable objects, like integers from the first example, will have the first example's result, True, but tuples are immutable and this happens:
a = (1,2)
b = (1,2)
a is b
False
Can someone explain please?
Thanks.
This question already has answers here:
Concatenating two lists - difference between '+=' and extend()
(11 answers)
Python append() vs. + operator on lists, why do these give different results?
(7 answers)
Closed 6 years ago.
Python lists have a += operator as well as append and extend methods.
If l is a list, is l += ... equivalent to l.append(...), l.extend(...), both, or neither?
In python += on a list is equivalent to extend method on that list.
This question already has answers here:
Is there a difference between "==" and "is"?
(13 answers)
Closed 9 years ago.
This might be a stupid question, but what exactly is the is function, and when would one use it?
From the context, i guess i could infer that it's equivalent to ==; but if that's the case, why have both? The Built-in Functions Reference shows nothing, and help(is) returns a SyntaxError.
is checks if the objects have the same identity. == only checks if they are equal.
>>> L1 = [1,2,3]
>>> L2 = [1,2,3]
>>> L1 is L2
False
>>> L1 == L2
True