Master Python Defaults, Closures, and Class Variables: Common Pitfalls Explained

1. The function def extendList(val, list=[]): demonstrates that a mutable default argument is created only once at function definition, causing subsequent calls without an explicit list to share the same list. The output is list1 = [10, 'a'], list2 = [123], list3 = [10, 'a']. The correct fix is to use def extendList(val, list=None): and create a new list inside the function.

2. The lambda‑based multipliers() suffers from Python’s closure late‑binding: all returned functions capture the same final value of i, so [m(2) for m in multipliers()] yields [6, 6, 6, 6]. Solutions include using a generator, binding the current value as a default argument ( lambda x, i=i: i*x), or employing functools.partial.

3. Class variables are stored in the class dictionary. Accessing Parent.x, Child1.x, Child2.x shows inheritance rules: modifying a child’s attribute masks the parent’s value, while changing the parent’s attribute affects children that have not overridden it, producing the sequence 1 1 1, 1 2 1, 3 2 3.

4. In Python 2 the / operator performs integer division for two integers, while // always performs floor division. The example prints 5/2 = 2, 5.0/2 = 2.5, 5//2 = 2, 5.0//2.0 = 2.0. In Python 3 / is true division, so the results become 5/2 = 2.5 and 5.0/2 = 2.5, while // still yields floor division.

4. (continued) The from __future__ import division statement can make Python 2 behave like Python 3 for /.

5. Slicing a list with a start index beyond its length returns an empty list instead of raising IndexError, e.g., list[10:] yields [].

6. Multiplying a list of mutable objects creates references to the same object. list = [[]] * 5 produces five references to the same inner list, so appending to one element appears in all, and list.append(30) adds a new element to the outer list, resulting in [[10, 20], [10, 20], [10, 20], [10, 20], [10, 20], 30].

7. A single list comprehension can select even numbers that also occupy even positions: [x for x in lst[::2] if x % 2 == 0], which on the example list yields [10, 18, 78].

8. Defining a subclass of dict with an __missing__ method that returns an empty list allows missing keys to be handled without error, as demonstrated by DefaultDict() where accessing an absent key returns [] instead of raising KeyError.