Question

I’ve run into a piece of code that reads:

queue = [(Xi, Xk) for Xi in csp.vars for Xk in csp.neighbors[Xi]]

Is this the equivalent of:

for Xi in csp.vars:
    for Xk in csp.neighbors[Xi]:
        queue.append((Xi, Xk))

or does this indicate something other than nested for statements?

Answer 1

As other already pointed out that’s exactly what it means.

I also find sometimes confusing using multiple for inside a list-comprehension/genexp, so I usually avoid them. When I use them I usually put every for in a different line, such as:

queue = [(Xi, Xk) for Xi in csp.vars
                  for Xk in csp.neighbors[Xi]]

Or even indenting:

queue = [(Xi, Xk) for Xi in csp.vars
                      for Xk in csp.neighbors[Xi]]

This makes it clear which is the inner for loop.

Also, list-comprehension can have an if expression for every for loop, so the complete syntax is something like:

queue = [(Xi, Xk) for Xi in csp.vars
                      if condition_one()
                          for Xk in csp.neighbors[Xi]
                              if condition_two()
                                  ... ]

Even though I hope you’ll never write something like that. The if expression at the end is fine, but mixing it with for makes all too cumbersome and not at all readable.