+- Python Forum (https://python-forum.io)
+-- Forum: Python Coding (https://python-forum.io/forum-7.html)
+--- Forum: General Coding Help (https://python-forum.io/forum-8.html)
+--- Thread: What is the reason for this difference between lists and tuples? (/thread-5251.html)
What is the reason for this difference between lists and tuples? - Athenaeum - Sep-25-2017
I am a Python beginner, and recently I discovered a key difference between lists and tuples.
If I write something like:
list1 = list1.append(list2)...then list1 becomes a "NoneType" object. In other words if you pass an appended list to its own name, then it is replaced with a NoneType object.
This isn't the case with tuples though...is there a reason for this?
RE: What is the reason for this difference between lists and tuples? - buran - Sep-25-2017
first of all tuple has no attribute append.
>>> x = (1,2) >>> y=(3,4) >>> x.append(y) Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'tuple' object has no attribute 'append'list1.append(list2) will append list2 the to the list1 in-place, i.e. what you should do
>>> list1=[1,2] >>> list2=[3,4] >>> list1.append(list2) >>> list1 [1, 2, [3, 4]]as you can see list2 is added as 3-rd element in the list1.
what I think you want to do is to use extend, instead of append
>>> list1=[1,2] >>> list2=[3,4] >>> list1.extend(list2) >>> list1 [1, 2, 3, 4]
RE: What is the reason for this difference between lists and tuples? - Athenaeum - Sep-25-2017
Sorry, I meant that this difference occurs when you use the __add__ method on a tuple, which I consider to be the equivalent of append for tuples.
RE: What is the reason for this difference between lists and tuples? - buran - Sep-25-2017
__add__ is internal implementation that is called by + operator (i.e. concatenation)
https://docs.python.org/3/library/stdtypes.html#typesseq
read carefully note 6 and 3rd item in the bullet list
Quote:Concatenating immutable sequences always results in a new object. This means that building up a sequence by repeated concatenation will have a quadratic runtime cost in the total sequence length. To get a linear runtime cost, you must switch to one of the alternatives below:
if concatenating str objects, you can build a list and use str.join() at the end or else write to an io.StringIO instance and retrieve its value when complete
if concatenating bytes objects, you can similarly use bytes.join() or io.BytesIO, or you can do in-place concatenation with a bytearray object. bytearray objects are mutable and have an efficient overallocation mechanism
if concatenating tuple objects, extend a list instead
for other types, investigate the relevant class documentation
apart from performance issue that is discussed, note that concatenating tuple is equivalent of extending a list, but extending the list has better performance