将列表转换为字符串列表python

Question

I'm Python beginner can't get my head around. How do I change for example a = [[1,2],[3,4],[5,6]] into "12\\n34\\n56" string format.

This is as far as i got but it goes into newline with each number.

def change(a):
    c = ""
    for r in a:
        for b in r:
            c += str(b) + "\n"
    return c

Answer 1

but it goes into newline with each number

That's because you add the newline after each number instead of after each sublist r . If you want that instead, you should append the newline there:

c = ''
for r in a:
    for b in r:
        c += str(b)
    c += '\n'
return c

But note that appending to a string is very inefficient, as it ends up creating lots of intermediary strings. Usually, you would create a list instead to which you append your string parts, and then finally join that list to convert it to a single string:

c = []
for r in a:
    for b in r:
        c.append(str(b))
    c.append('\n')
return ''.join(c)

And then, you can also use list expressions to make this shorter in multiple steps; first for the inner list:

c = []
for r in a:
    c.extend([str(b) for b in r])
    c.append('\n')
return ''.join(c)

And you can join that list comprehension first:

c = []
for r in a:
    c.append(''.join([str(b) for b in r]))
    c.append('\n')
return ''.join(c)

Then you can move the newline into the outer join, and make a new list comprehension for the outer list:

c = [''.join([str(b) for b in r]) for r in a]
return '\n'.join(c)

And at that point, you can make it a one-liner too:

return '\n'.join([''.join([str(b) for b in r]) for r in a])

As Padraic pointed out in the comments, joining on the newline character will also prevent the string from having a trailing \\n which you would end up if you kept adding it in the loop. Otherwise, you could have used str.rstrip('\\n') to get rid of it afterwards.

Answer 2

Using str.join with generator expression :

>>> a = [[1,2], [3,4], [5,6]]
>>> '\n'.join(''.join(map(str, xs)) for xs in a)
'12\n34\n56'