How to check if this deque is empty in Python?

Question

from collections import deque 
Q = deque()

How to check if this deque is empty? Is there any function like isEmpty() to check this? Can someone help? I looked for a function in the documentation but unable to find any?

you can use len(Q) https://docs.python.org/3/library/functions.html#len — Anurag Wagh, Jun 05 '20 at 10:25
Do normal operations of list also works on it? like q[0] and q[-1]? — Ritika Gupta, Jun 05 '20 at 10:26
@A.J.Uppal I checked it but it hasn't mentioned it is for "from collections import deque " this queue or general queue (list implementation) — Ritika Gupta, Jun 05 '20 at 10:28
@RitikaGupta I believe for `collections.deque` it works: https://bitbucket.org/pypy/pypy/src/default/lib_pypy/_collections.py — A.J. Uppal, Jun 05 '20 at 10:30
*Any* collection evaluates to false when empty. ``if Q:`` is totally sufficient to check whether it is not empty, and ``if not Q:`` to check whether it is empty. — MisterMiyagi, Jun 05 '20 at 10:42

score 1 · Answer 1 · answered Jun 05 '20 at 10:42

Simply pythonic way:

from collections import deque 
Q = deque()
if not Q:
   print("Queue is empty")

You should know this power of great Python, that each collection becomes false if it is empty let it be sets, lists, dictionary, deque etc.

if data_structure:
    print('Data structure is not empty')
else:
   print('Data structure is empty')

mangelfdz · Accepted Answer · 2020-06-05T10:36:42.610

0

The length of Q will be 0 if the input of deque() is empty:

from collections import deque
Q = deque()
assert len(Q) == 0

edited Jun 05 '20 at 10:36

answered Jun 05 '20 at 10:28

mangelfdz

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`__len__` simply invokes `len`, which is far more pythonic. `__len__` is a magic method. – A.J. Uppal Jun 05 '20 at 10:32
yes, you are right, that's easier – mangelfdz Jun 05 '20 at 10:36
@A.J.Uppal: You got it backwards; `len` is implemented in terms of `__len__`; that said, `len` is still the correct solution (it can access `__len__` through optimized C level interfaces, and it's just generally considered more Pythonic to use the special methods implicitly, not explicitly). – ShadowRanger Jun 07 '20 at 13:36

How to check if this deque is empty in Python?

2 Answers2