I am somehow familiar with list comprehensions in Python. But in situations when I need to check if the list is not None, the list comprehension would fail.
e.g.
tags = v.tags if tags: for t in tags: if t['Key'] == 'Name': # Do something Now, if tags is None, then the following list comprehension fails. It works fine if tags is empty/[]. I would like a list comprehension that checks against None.
[k for k,v in tags if tags]: How about this:
[k for k in (tags or [])] Let's see what happens for both cases:
>>> tags = None
>>> [k for k in (tags or [])] [] tags = [1, 2, 3]
>>> [k for k in (tags or [])] [1, 2, 3] The reason this works is because (tags or []) returns tags only if bool(tags) == True. Otherwise it returns the second argument, in this case [], even if its boolean value is also False. That way, we either loop over tags, if it exists` or over an empty list if it doesn't.
You can use a ternary condition here:
([k for k, v in tags] if tags is not None else []) You can embed the ternary condition in the comprehension as well:
[k for k, v in (tags if tags is not None else [])] As a side note, [k for k, v in tags if tags] does not really behave as you might expect. The if clause of a list comprehension is evaluated at each iteration, meaning that the truth value of tags is checked for each element in it.
To prove this:
l = [1, 2, 3, 4, 5] def is_empty(l): print("is_empty") return len(l) > 0 m = [i for i in l if is_empty(l)] print(m) Output:
is_empty is_empty is_empty is_empty is_empty [1, 2, 3, 4, 5] What you are semantically looking for is an inline if, that is, in Python, a ternary condition.
You can test with a preceding if block
if tags is not None: ... Or short-circuit with an empty iterable:
>>> tags = None >>> [k for k, _ in tags or ()] [] 
tags.. You can do this with empty collectors ([],(),{}) but not overNone.[k for k,v in (tags or [])]