Inconsistency of Python plus operator

Question

In [26]: l=[]

In [27]: s="asdsad"

In [28]: l+=s

In [29]: l
Out[29]: ['a', 's', 'd', 's', 'a', 'd']

However,

In [30]: l+s
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)

/home/xiaohan/code/proteincrawler/id_crawler.py in <module>()
----> 1 
      2 
      3 
      4 
      5 

TypeError: can only concatenate list (not "str") to list

So, the + operators in '+=' and '+' are different.

But I think they should be the same, because they are all plus

Am I wrong or something happens behind the scene?

The only difference that "pop up" from your post is that, in the second plus operator, you didn't assign the result to a variable. That's translate with the operation: print(l+s) and, maybe, this messes up the whole thing. Have you tried yet to assign l+s to a new variable ? — DonCallisto
– DonCallisto, Commented Sep 28, 2011 at 7:42

taskinoor · Accepted Answer · 2011-09-28 09:05:14Z

7

This is explained here. Copied from the link:

It's because the += operator is doing the equivalent of calling the extend method, which treats its argument as a generic sequence, and doesn't enforce type.

edited Sep 28, 2011 at 9:05

answered Sep 28, 2011 at 7:40

taskinoor

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