Names, mutability and copies

Names point at objects

a = [1, 2]
b = a            # same list, two names
b.append(3)
a                # [1, 2, 3]

Assignment never copies. Passing a list into a function passes the same list: if the function mutates it, the caller sees the change.

def add_tag(tags, t):
    tags.append(t)       # mutates the caller's list
    return tags

def add_tag(tags, t):
    return [*tags, t]    # returns a new list

Shallow vs deep copy

import copy
orig = {"user": {"name": "a"}, "tags": ["x"]}
shallow = dict(orig)            # or orig.copy(), {**orig}
shallow["tags"].append("y")     # orig["tags"] changed too: inner objects are shared
deep = copy.deepcopy(orig)      # fully independent

list(x), x[:], x.copy() and {**d} are all shallow.

+= on lists mutates

a = [1]; b = a
a += [2]         # in place: b is [1, 2] too
a = a + [3]      # new list: b unaffected

is vs ==

• == compares values. is compares identity.
• Use is only for None (and other singletons): if x is None.
• a is b for small ints or short strings sometimes "works" because of caching. Don't rely on it.

Mutable vs immutable

Only hashable (immutable) values can be dict keys or set members. A tuple containing a list isn't hashable. Use tuple(sorted(items)) or frozenset(items) as a key.

Class attributes are shared

class Account:
    history = []                 # ❌ ONE list for every account
    def __init__(self):
        self.history = []        # ✅ per instance