answer:Here's one possible implementation of the common_elements function: ```python def common_elements(list1, list2): if not isinstance(list1, list) or not isinstance(list2, list): raise TypeError("Inputs must be lists") common = [] for elem in list1: if elem in list2: common.append(elem) return common ``` This function uses the `isinstance` function to check if the inputs are lists. If either input is not a list, the function raises a `TypeError` with a descriptive message. The function then initializes an empty list called `common`. It loops through every element in `list1` and checks if that element is also in `list2`. If it is, the element is appended to `common`. Once the loop is finished, the function returns `common`. If either input list was empty, `common` will also be empty. The time complexity of this function is O(N*M), where N and M are the lengths of the input lists; however, using the `set` function can make it faster: ```python def common_elements(list1, list2): if not isinstance(list1, list) or not isinstance(list2, list): raise TypeError("Inputs must be lists") set1, set2 = set(list1), set(list2) common = list(set1.intersection(set2)) return common ``` This version of the function first converts the input lists to sets using the `set` function. It then takes the intersection of the two sets using the `intersection` method and converts the resulting set back to a list using the `list` function. The advantage of this approach is that set intersection is a much faster operation than looping through lists. The time complexity of this function is O(N+M) for converting the lists to sets and O(min(N,M)) for computing the intersection, so the overall time complexity is still O(N+M).

answer:Certainly! Here's the code again, this time with comments explaining each part: ```python def common_elements(list1, list2): # Check that both inputs are lists if not isinstance(list1, list) or not isinstance(list2, list): raise TypeError("Inputs must be lists") # Initialize an empty list to hold the common elements common = [] # Loop through each element in list1 for elem in list1: # Check if the element is also in list2 if elem in list2: # If it is, add it to the common list common.append(elem) # Return the list of common elements return common ``` The code uses `isinstance` to check that both inputs are lists. If either one is not a list, it raises a `TypeError` with an informative message. Next, it initializes an empty list called `common` which will hold the common elements found in both lists. The loop then goes through each element in `list1`. For each element, it checks if that element is also in `list2`. If it is, the element is added to the `common` list. Finally, the function returns the `common` list, which contains all elements found in both lists. Overall, this function is a simple but effective way to find the common elements between two lists.

answer:Sure! Here are a few examples that demonstrate how the `common_elements` function works in different cases: ```python # Example 1: Two lists with common elements list1 = [1, 2, 3, 4, 5] list2 = [3, 4, 5, 6, 7] print(common_elements(list1, list2)) # Output: [3, 4, 5] # Example 2: Two lists with no common elements list1 = [1, 2, 3] list2 = [4, 5, 6] print(common_elements(list1, list2)) # Output: [] # Example 3: One empty list list1 = [] list2 = [1, 2, 3] print(common_elements(list1, list2)) # Output: [] # Example 4: Inputs are not lists list1 = "not a list" list2 = [1, 2, 3] print(common_elements(list1, list2)) # Raises TypeError: "Inputs must be lists" ``` In Example 1, the function correctly identifies the common elements between `list1` and `list2`, which are the numbers 3, 4, and 5. In Example 2, the function returns an empty list because there are no common elements between the two lists. In Example 3, since one list is empty, the function returns an empty list. In Example 4, the function raises a `TypeError` because at least one of the inputs is not a list.

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