Swap the square brackets for parentheses in the return statement and it will return a generator expression.

That will read the file as needed (ie as you iterate over it) instead of loading the entire thing in memory.

    for record in read_portfolio(fn):
        # do stuff

If you do that, it will try to read from a closed file.

Or even:

  def read_portfolio(filename):
      with open(filename) as f:
          rows = csv.reader(f)
          headers = next(rows)
          return [
              {
                  "name": r[0],
                  "shares": int(r[1]),
                  "price": float(r[2]),
              }
              for r in rows
          ]