Working with Python Dict and Frozendict

Python
# Mutable mapping
d = {"a": 1, "b": 2}
d["c"] = 3
d.update(x=10)

# Immutable mapping (frozendict, added as a builtin in newer Python)
fd = frozendict({"a": 1, "b": 2})
# fd["a"] = 99  # raises TypeError: object does not support item assignment

# Convert between them
d2 = dict(fd)
fd2 = frozendict(d2)

Progress:

• Determine if the mapping needs to be mutable (dict) or immutable/hashable (frozendict)
• Choose construction method (literal, dict()/frozendict() constructor, comprehension, fromkeys)
• Use appropriate access patterns ([], .get(), .setdefault() for dict only)
• Iterate via .keys(), .values(), .items() view objects
• Apply merge/update via | and |= (dict) or | (frozendict, returns new frozendict)
• Use frozendict when a mapping must be a dict key, set member, or passed as an immutable default/config

1. Construction

Python
d = dict(a=1, b=2)
d = {k: v for k, v in pairs}
d = dict.fromkeys(["a", "b"], 0)

fd = frozendict(a=1, b=2)
fd = frozendict({"a": 1})
fd = frozendict(zip(keys, values))

2. Access and mutation

• Both support d[key], d.get(key, default), key in d, len(d).
• Only dict supports d[key] = value, del d[key], .pop(), .popitem(), .clear(), .update(), .setdefault().
• frozendict raises TypeError on any mutation attempt — treat it like tuple vs list.

3. Views and iteration

Python
for k, v in d.items():
    ...
keys_view = d.keys()      # dynamic view, reflects later dict changes
values_view = d.values()
items_view = d.items()    # set-like if values are hashable

frozendict also exposes .keys(), .values(), .items(), but views are effectively static since the underlying mapping can't change.

4. Merging

Python
merged = d1 | d2          # new dict
d1 |= d2                  # in-place update, dict only

merged_fd = fd1 | fd2     # new frozendict, no in-place |= (would require mutation)

5. Hashability

Python
config = frozendict(timeout=30, retries=3)
cache = {config: "result"}   # valid: frozendict is hashable if all values are hashable
# {d: "result"}               # invalid: dict is unhashable, raises TypeError