Good way to concatenate str to list?

Question

Is there a way to efficiently concatenate str and list?

inside = [] #a list of Items

class Backpack:
    def add(toadd):
        inside += toadd

print "Your backpack contains: " #now what do I do here?

Answer 1

It sounds like you're just trying to add a string to a list of strings. That's just append:

>>> inside = ['thing', 'other thing']
>>> inside.append('another thing')
>>> inside
['thing', 'other thing', 'another thing']

There's nothing specific here to strings; the same thing works for a list of Item instances, or a list of lists of lists of strings, or a list of 37 different things of 37 different types.

In general, append is the most efficient way to concatenate a single thing onto the end of a list. If you want to concatenate a bunch of things, and you already have them in a list (or iterator or other sequence), instead of doing them one at a time, use extend to do them all at once, or just += instead (which means the same thing as extend for lists):

>>> inside = ['thing', 'other thing']
>>> in_hand = ['sword', 'lamp']
>>> inside += in_hand
>>> inside
['thing', 'other thing', 'sword', 'lamp']

If you want to later concatenate that list of strings into a single string, that's the join method, as RocketDonkey explains:

>>> ', '.join(inside)
'thing, other thing, another thing'

I'm guessing you want to get a little fancier and put an "and" between the last to things, skip the commas if there are fewer than three, etc. But if you know how to slice a list and how to use join, I think that can be left as an exercise for the reader.

If you're trying to go the other way around and concatenate a list to a string, you need to turn that list into a string in some way. You can just use str, but often that won't give you what you want, and you'll want something like the join example above.

At any rate, once you have the string, you can just add it to the other string:

>>> 'Inside = ' + str(inside)
"Inside = ['thing', 'other thing', 'sword', 'lamp']"
>>> 'Inside = ' + ', '.join(inside)
'Inside = thing, other thing, another thing'

If you have a list of things that aren't strings and want to add them to the string, you have to decide on the appropriate string representation for those things (unless you're happy with repr):

>>> class Item(object):
...   def __init__(self, desc):
...     self.desc = desc
...   def __repr__(self):
...     return 'Item(' + repr(self.desc) + ')'
...   def __repr__(self):
...     return self.desc
...
>>> inside = [Item('thing'), Item('other thing')]
>>> 'Inside = ' + repr(inside)
... "Inside = [Item('thing'), Item('other thing')]"
>>> 'Inside = ' + str(inside)
... "Inside = [Item('thing'), Item('other thing')]"
>>> 'Inside = ' + ', '.join(str(i) for i in inside)
... 'Inside = thing, other thing'

Notice that just calling str on a list of Items calls repr on the individual items; if you want to call str on them, you have to do it explicitly; that's what the str(i) for i in inside part is for.

Putting it all together:

class Backpack:
    def __init__(self):
        self.inside = []
    def add(self, toadd):
        self.inside.append(toadd)
    def addmany(self, listtoadd):
        self.inside += listtoadd
    def __str__(self):
        return ', '.join(str(i) for i in self.inside)

pack = Backpack()
pack.add('thing')
pack.add('other thing')
pack.add('another thing')
print 'Your backpack contains:', pack

When you run this, it will print:

Your backpack contains: thing, other thing, another thing

Answer 2

You could try this:

In [4]: s = 'Your backpack contains '

In [5]: l = ['item1', 'item2', 'item3']

In [6]: print s + ', '.join(l)
Your backpack contains item1, item2, item3

The join method is a bit odd compared to other Python methods in its setup, but in this case it means 'Take this list and convert it to a string, joining the elements together with a comma and a space'. It is a bit odd because you specify the string with which to join first, which is a little outside of the ordinary but becomes second nature soon :) See here for a discussion.

If you are looking to add items to inside (a list), the primary way to add items to a list is to use the append method. You then use join to bring all the items together as a string:

In [11]: inside = []

In [12]: inside.append('item1')

In [13]: inside.append('item2')

In [14]: inside.append('item3')

In [15]: print 'Your backpack contains ' + ', '.join(inside)
Your backpack contains item1, item2, item3