Why am i not able to reduce the list by using -= where as I was able to add to the list using +=

Question

Similar to the way of adding new items to the list using += why does not python allow me to remove the items from a list using -=

Chris Freeman · Accepted Answer

The += syntax works because adding an item or another list to a list works. 
Trying to use the in-place subtract -= doesn't have a meaning when used with lists since subtracting one list from another is not allowed. Trying to do so will raise a TypeError:
Python
TypeError: unsupported operand type(s) for -: 'list' and 'list'
 
More on operators is in the docs (https://docs.python.org/3/library/operator.html).

Nathan Tallack · Answer

Now be patient with me on this one.  I am just as new to this as you.  :)
I know that a list is a builtin type for python.  And as most types do, it has it's own methods for manipulating its objects.
Consider the python docs (https://docs.python.org/3/tutorial/datastructures.html) for the methods that this type has.
Now, I am not sure about using the += and -= operators on a list type.  But one thing I am sure of with Object Orientated Programming is making use of the methods that your object comes for manipulating data within object.
Now I believe the + operator for lists overloads on the extend() method.  I think.  May be wrong on this.  But the outcomes do look the same.  If I was a little smarter with python I could look at the class definition for list type and understand it, but that is a little beyond me right now.
So consider what you are trying to do, and determine if one of those methods listed would be suitable for your requirements.  :)

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ravi thotapalli

ravi thotapalli

Why am i not able to reduce the list by using -= where as I was able to add to the list using +=

2 Answers

Chris Freeman

Chris Freeman

Nathan Tallack

Nathan Tallack