The error message:
IndexError: deque index out of range
occurs when you try to access or remove an element from a deque (double-ended queue) at an invalid index. The deque is part of Python’s collections module and is used for efficient appends and pops from both ends.
1. Causes and Solutions
Cause 1: Accessing an Invalid Index in deque
A deque behaves like a list, but if you try to access an index that does not exist, Python raises an IndexError.
Incorrect Code:
from collections import deque
dq = deque([10, 20, 30])
print(dq[3]) # Error: Index 3 does not exist (valid indices: 0, 1, 2)
Solution: Ensure the index is within the valid range.
if len(dq) > 3:
print(dq[3])
else:
print("Index is out of range")
Cause 2: Using popleft() or pop() on an Empty deque
The popleft() method removes the first element, and pop() removes the last element. If the deque is empty, calling these methods raises an IndexError.
Incorrect Code:
dq = deque()
dq.popleft() # Error: No elements to remove
Solution: Check if the deque is non-empty before calling popleft() or pop().
if dq:
dq.popleft()
else:
print("Deque is empty, cannot pop")
Cause 3: Using an Out-of-Range Negative Index
Negative indices refer to elements from the end, but if they exceed the valid range, an error occurs.
Incorrect Code:
dq = deque([10, 20, 30])
print(dq[-4]) # Error: Only -1, -2, and -3 are valid
Solution: Ensure the negative index is within range.
if -4 >= -len(dq):
print(dq[-4])
else:
print("Negative index out of range")
Cause 4: Removing Too Many Elements in a Loop
If you continuously remove elements from a deque without checking its size, you may try to remove an element from an empty deque, causing an error.
Incorrect Code:
dq = deque([1, 2, 3])
for _ in range(5): # Error: deque has only 3 elements
dq.popleft()
Solution: Use a while loop to check if elements exist before removing them.
while dq:
dq.popleft()