The rule of thumb is to avoid using more than two expressions in a list comprehension. This could be two conditions, two loops, or one condition and one loop. As soon as it gets more complicated than that, you should use normal if and for statements and write a helper function

Dictionaries and sets have their own equivalents of list comprehen-sions.

d = { "foo": "bar", "baz": "qux" }
r = {val: key for key, val in d.items()}
#=> { "bar": "foo", "qux": "baz" }

Unlike map, list comprehensions let you easily filter items from the input list, removing corresponding outputs from the result.

Beware that indexing a list by a negative variable is one of the few situations in which you can get surprising results from slicing. For example, the expression so melist[-n:] will work fine when n is greater than one (e.g., somelist[-3:]). However, when n is zero, the expression so melist[-0:] will result in a copy of the original list.

Slicing can be extended to any Python class that implements the__getitem__ and __setitem__ special methods

In a file, functions and classes should be separated by two blank lines.