How do you check if stack is empty or not in python?
Short Answer:Place the list in a boolean context (for example, with an
PEP 8PEP 8, the official Python style guide for Python code in Python's standard library, asserts:
We should expect that standard library code should be as performant and correct as possible. But why is that the case, and why do we need this guidance? ExplanationI frequently see code like this from experienced programmers new to Python:
And users of lazy languages may be tempted to do this:
These are correct in their respective other languages. And this is even semantically correct in Python. But we consider it un-Pythonic because Python supports these semantics directly in the list object's interface via boolean coercion. From the docs (and note specifically the inclusion of the
empty list,
And the datamodel documentation:
and
So instead of this:
or this:
Do this:
Doing what's Pythonic usually pays off in performance:Does it pay off? (Note that less time to perform an equivalent operation is better:)
For scale, here's the cost of calling the function and constructing and returning an empty list, which you might subtract from the costs of the emptiness checks used above:
We see that either checking for length
with the builtin function Why? For the First Python has to check the globals to see if Then it must call the function, load
And for the
The "Pythonic" way is a much simpler and faster check since the length of the list is cached in the object instance header:
Evidence from the C source and documentation
From the c source in Include/listobject.h:
Response to comments:
IPython magic,
We can see there's a bit of linear cost for each additional
Now let's look at the case for an unempty list:
What we can see here is that it makes little difference whether you pass in an actual Python is written in C; it uses its logic at the C level. Anything you write in Python will be slower. And it will likely be orders of magnitude slower unless you're using the mechanisms built into Python directly. How do you check if an object is empty in Python?You can check if the list is empty in python using the bool() function. bool() function returns boolean value of the specified object. The object will always return True , unless the object is empty, like [] , () , {} . You can use the bool() function for any of the list-like objects.
How do you check for an empty array in Python?How to check if a NumPy array is empty in Python. empty_array = np. array([]). is_empty = empty_array. size == 0.. print(is_empty). nonempty_array = np. array([1, 2, 3]). is_empty = nonempty_array. size == 0.. print(is_empty). How do I know if a nested list is empty?Use the any() function. This returns True if any list within the list is not empty. alist = [[],[]] if not any(alist): print("Empty list!") >> Empty list!
How do you check for null in Python?Use the is operator to check if a variable is null in Python, e.g. if my_var is None: . The is operator returns True if the values on the left-hand and right-hand sides point to the same object and should be used when checking for singletons like None .
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