Question

I have a list of objects. In a single line I would like to create a string that contains a specific variable of each object in the list, separated by commas.

Right now I’m able to achieve this using a combination of list comprehensions and map like so:

','.join(map(str, [instance.public_dns_name for instance in instances]))

or using lambda:

','.join(map(str, [(lambda(i): i.public_dns_name)(instance) for instance in instances]))

Each instance object has a “public_dns_name” variable that returns the host name. This returns a string like this:

host1,host2,hos3,host4

Is it possible to achieve the same thing using only the list comprehension?

Answer 1

You can’t use just a list comprehension, you’ll still need to use join.

It’s more efficient to use a generator expression

','.join(str(instance.public_dns_name) for instance in instances)

A list comprehension would look like this:

','.join([str(instance.public_dns_name) for instance in instances])

The difference is that it creates the entire list in memory before joining, whereas the generator expression will create the components as they are joined