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Muiduzzaman Lipu for Refactorly

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Python 3.15 Is Out: 5 New Features You'll Actually Use (Tested)

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Python 3.15 is out, with a new keyword, a new built-in dictionary type, and a profiler that can look inside a running program. I tested every feature below on Python 3.15.2, and the outputs are real.

Video version (3 minutes):

1. lazy import: load modules only when you use them 

lazy import json
import sys

print("json" in sys.modules)   # False: not loaded yet
json.dumps({"ok": True})       # first use → loads now
print("json" in sys.modules)   # True
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lazy import json doesn't load the module yet. The moment you actually use json, it loads. You keep normal-looking imports at the top of the file, but you only pay for the ones you use. lazy from decimal import Decimal works too.

Does it matter? I wrote a script that imports 10 standard-library modules (asyncio, http.server, sqlite3, email, …) but only prints hello, and timed 15 runs of each version:

normal imports:  median startup 68.9 ms
lazy imports:    median startup 14.6 ms   (~4.7× faster)
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That's a big deal for CLI tools, where most commands only need a few modules.

Two rules I hit while testing:

  • lazy import is only allowed at module level. Inside a function you get SyntaxError: lazy import not allowed inside functions.
  • It's new syntax, so older Pythons fail with SyntaxError: invalid syntax.

2. frozendict: an immutable, hashable dict 

config = frozendict(host="localhost", port=8080)

config["port"] = 9000
# TypeError: 'frozendict' object does not support item assignment

cache = {config: "connected"}       # ✅ works as a dict key

config | {"port": 9000}
# frozendict({'host': 'localhost', 'port': 9000})  (a new copy)
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Because it can't change, a frozendict is hashable: you can use it as a dict key or put it in a set. A normal dict gives TypeError: cannot use 'dict' as a dict key (unhashable type: 'dict'). Note that frozendict is not a subclass of dict (isinstance(config, dict) is False).

It's a good fit for configuration and constants.

3. * and ** inside comprehensions 

rows = [[1, 2], [3], [4, 5, 6]]
[*row for row in rows]
# [1, 2, 3, 4, 5, 6]

dicts = [{"a": 1}, {"b": 2}, {"a": 3}]
{**d for d in dicts}
# {'a': 3, 'b': 2}
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Flattening a list of lists no longer needs a double for or itertools.chain. ** merges a list of dicts, and later keys win, just like normal dict merging.

4. sentinel(): a clean "no value given"

Sometimes None is a valid value, so it can't also mean "nothing was passed". People used to write MISSING = object(). Now there's a built-in:

MISSING = sentinel("MISSING")

def get(key, default=MISSING):
    if default is MISSING:
        return "no default given"
    return default

get("x")         # 'no default given'
get("x", None)   # None  (None is a real value here!)
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It has a readable name in reprs and error messages (MISSING instead of <object object at 0x…>).

5. Tachyon: a built-in sampling profiler 

$ python -m profiling.sampling run slow.py

  sample%  tottime (ms)  function
     96.0       242.000  slow.py:7(slow_search)  🔥
      3.2         8.000  slow.py:3(load_data)
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It samples what your program is doing thousands of times per second with very low overhead. On my test script it immediately showed that 96% of the time was in one function. It can also attach to a process that's already running:

python -m profiling.sampling attach <PID>
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It can produce flame graphs too (--flamegraph -o out.html).

⚠️ macOS note: it needs elevated permissions there (I got "Unable to access process memory on macOS"), so I ran this example in a Linux container (python:3.15-slim with --cap-add SYS_PTRACE).

Recap

  • lazy import → faster startup (69 ms → 15 ms in my test)
  • frozendict → immutable, hashable dict
  • [*x for x in …] → flatten in one line
  • sentinel() → a "no value" marker that isn't None
  • profiling.sampling → find slow code, even in running apps

There's more in the official release notes, such as UTF-8 as the default encoding. Which feature will you use first?

I post one tested coding tip every day on YouTube at Refactorly. The 30-second version of this one is here.

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