{"id":170,"date":"2026-10-05T14:00:14","date_gmt":"2026-10-05T14:00:14","guid":{"rendered":"https:\/\/devdojo.co.in\/?p=170"},"modified":"2026-10-05T14:00:14","modified_gmt":"2026-10-05T14:00:14","slug":"python-3-15-new-features","status":"publish","type":"post","link":"https:\/\/devdojo.co.in\/index.php\/2026\/10\/05\/python-3-15-new-features\/","title":{"rendered":"Python 3.15 New Features: Lazy Imports, frozendict &#038; More"},"content":{"rendered":"\n<div class=\"dd-post\">\n<style>\n.single article:has(.dd-hero) .entry-media{display:none}\n.dd-post{--dd-navy:#0b1020;--dd-navy2:#151b3b;--dd-blue:#3178c6;--dd-violet:#7c5cff;--dd-code:#0f1430;--dd-text:#1f2937;--dd-muted:#5b6474;--dd-line:#e3e7ef;font-size:17px;line-height:1.75;color:var(--dd-text)}\n.dd-post h2{font-size:1.65em;line-height:1.3;margin:2.2em 0 .6em;padding-bottom:.3em;border-bottom:3px solid var(--dd-violet);scroll-margin-top:90px}\n.dd-post h3{font-size:1.25em;margin:1.6em 0 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id=\"ddHeroTitle\">Python 3.15 New Features<\/title>\n<desc id=\"ddHeroDesc\">Animated banner showing the main Python 3.15 features: lazy imports, frozendict, unpacking in comprehensions, UTF-8 by default, the Tachyon sampling profiler and the sentinel built-in, all connected to a central Python 3.15 node.<\/desc>\n<defs>\n<linearGradient id=\"ddBg\" x1=\"0\" y1=\"0\" x2=\"1\" y2=\"1\"><stop offset=\"0\" stop-color=\"#0b1020\"\/><stop offset=\"1\" stop-color=\"#151b3b\"\/><\/linearGradient>\n<linearGradient id=\"ddAcc\" x1=\"0\" y1=\"0\" x2=\"1\" y2=\"0\"><stop offset=\"0\" stop-color=\"#3178c6\"\/><stop offset=\"1\" stop-color=\"#7c5cff\"\/><\/linearGradient>\n<radialGradient id=\"ddGlow\" cx=\"0.5\" cy=\"0.5\" r=\"0.5\"><stop offset=\"0\" stop-color=\"#7c5cff\" stop-opacity=\"0.55\"\/><stop offset=\"1\" stop-color=\"#7c5cff\" stop-opacity=\"0\"\/><\/radialGradient>\n<\/defs>\n<rect class=\"dd-bg\" x=\"0\" y=\"0\" width=\"1200\" height=\"520\" fill=\"url(#ddBg)\"\/>\n<g opacity=\"0.12\" stroke=\"#7c8cff\" stroke-width=\"1\">\n<path d=\"M0 104H1200M0 208H1200M0 312H1200M0 416H1200M150 0V520M300 0V520M450 0V520M600 0V520M750 0V520M900 0V520M1050 0V520\"\/>\n<\/g>\n<circle cx=\"900\" cy=\"260\" r=\"210\" fill=\"url(#ddGlow)\"\/>\n<text x=\"64\" y=\"118\" fill=\"#9fb3ff\" font-family=\"Segoe UI, Roboto, Helvetica, Arial, sans-serif\" font-size=\"22\" font-weight=\"600\" letter-spacing=\"4\">DEVDOJO \u00b7 BACKEND \u00b7 PYTHON<\/text>\n<text x=\"64\" y=\"196\" fill=\"#ffffff\" font-family=\"Segoe UI, Roboto, Helvetica, Arial, sans-serif\" font-size=\"64\" font-weight=\"800\">Python 3.15<\/text>\n<text x=\"64\" y=\"268\" fill=\"#ffffff\" font-family=\"Segoe UI, Roboto, Helvetica, Arial, sans-serif\" font-size=\"52\" font-weight=\"800\">New Features<\/text>\n<rect x=\"64\" y=\"292\" width=\"220\" height=\"6\" rx=\"3\" fill=\"url(#ddAcc)\"\/>\n<text x=\"64\" y=\"346\" fill=\"#c3c9e8\" font-family=\"Segoe UI, Roboto, Helvetica, Arial, sans-serif\" 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import<\/text><\/g>\n<g class=\"node\"><rect x=\"980\" y=\"98\" width=\"140\" height=\"44\" rx=\"12\" fill=\"#1c2350\" stroke=\"#3178c6\" stroke-width=\"2\"\/><text x=\"1050\" y=\"126\" fill=\"#cfd6ff\">frozendict<\/text><\/g>\n<g class=\"node\"><rect x=\"1040\" y=\"238\" width=\"130\" height=\"44\" rx=\"12\" fill=\"#1c2350\" stroke=\"#3178c6\" stroke-width=\"2\"\/><text x=\"1105\" y=\"266\" fill=\"#cfd6ff\">UTF-8<\/text><\/g>\n<g class=\"node\"><rect x=\"980\" y=\"378\" width=\"140\" height=\"44\" rx=\"12\" fill=\"#1c2350\" stroke=\"#3178c6\" stroke-width=\"2\"\/><text x=\"1050\" y=\"406\" fill=\"#cfd6ff\">Tachyon<\/text><\/g>\n<g class=\"node\"><rect x=\"690\" y=\"378\" width=\"140\" height=\"44\" rx=\"12\" fill=\"#1c2350\" stroke=\"#3178c6\" stroke-width=\"2\"\/><text x=\"760\" y=\"406\" fill=\"#cfd6ff\">sentinel<\/text><\/g>\n<g class=\"node\"><rect x=\"630\" y=\"238\" width=\"140\" height=\"44\" rx=\"12\" fill=\"#1c2350\" stroke=\"#3178c6\" stroke-width=\"2\"\/><text x=\"700\" y=\"266\" fill=\"#cfd6ff\">[*it for \u2026]<\/text><\/g>\n<\/g>\n<circle cx=\"900\" cy=\"260\" r=\"62\" fill=\"#0f1430\" stroke=\"url(#ddAcc)\" stroke-width=\"5\"\/>\n<circle cx=\"900\" cy=\"260\" r=\"62\" fill=\"none\" stroke=\"#7c5cff\" stroke-width=\"2\" opacity=\"0.6\"><animate attributeName=\"r\" values=\"62;84;62\" dur=\"3s\" repeatCount=\"indefinite\"\/><animate attributeName=\"opacity\" values=\"0.6;0;0.6\" dur=\"3s\" repeatCount=\"indefinite\"\/><\/circle>\n<text x=\"900\" y=\"272\" text-anchor=\"middle\" fill=\"#ffffff\" font-family=\"Segoe UI, Roboto, Helvetica, Arial, sans-serif\" font-size=\"34\" font-weight=\"800\">3.15<\/text>\n<\/svg>\n<\/div>\n\n<p><strong>Python 3.15 new features<\/strong> are finally landing in a stable release: the final version is scheduled for <strong>October 9, 2026<\/strong>, and Release Candidate 3 is already out. This is one of the most developer-friendly Python releases in years. You get <em>explicit lazy imports<\/em> for faster startup, a built-in <code>frozendict<\/code>, unpacking inside comprehensions, UTF-8 as the default text encoding, a brand-new sampling profiler called Tachyon, and a proper <code>sentinel<\/code> built-in. In this guide we will go through each feature with small, runnable examples, look at what can break when you upgrade, and finish with a practical upgrade checklist for your backend projects.<\/p>\n<div class=\"dd-note\"><b>What is a PEP?<\/b> A PEP (Python Enhancement Proposal) is the official design document for a new Python feature. Each one has a number \u2014 for example, <a href=\"https:\/\/peps.python.org\/pep-0810\/\" target=\"_blank\" rel=\"noopener\">PEP 810<\/a> is the proposal for lazy imports. Click any PEP number in this article to read the original proposal.<\/div>\n\n<div class=\"dd-tldr\">\n<strong class=\"dd-label\">Quick Summary<\/strong>\n<ul>\n<li><b>Lazy imports (<a href=\"https:\/\/peps.python.org\/pep-0810\/\" target=\"_blank\" rel=\"noopener\">PEP 810<\/a>):<\/b> write <code>lazy import json<\/code> and the module loads only when you first use it. Great for CLIs and big apps.<\/li>\n<li><b>frozendict (<a href=\"https:\/\/peps.python.org\/pep-0814\/\" target=\"_blank\" rel=\"noopener\">PEP 814<\/a>):<\/b> an immutable, hashable dict \u2014 use it as a cache key or a safe config object.<\/li>\n<li><b>Unpacking in comprehensions (<a href=\"https:\/\/peps.python.org\/pep-0798\/\" target=\"_blank\" rel=\"noopener\">PEP 798<\/a>):<\/b> <code>[*row for row in rows]<\/code> flattens lists in one clean line.<\/li>\n<li><b>UTF-8 by default (<a href=\"https:\/\/peps.python.org\/pep-0686\/\" target=\"_blank\" rel=\"noopener\">PEP 686<\/a>):<\/b> <code>open()<\/code> now uses UTF-8 everywhere, including Windows.<\/li>\n<li><b>Tachyon profiler (<a href=\"https:\/\/peps.python.org\/pep-0799\/\" target=\"_blank\" rel=\"noopener\">PEP 799<\/a>):<\/b> <code>python -m profiling.sampling run app.py<\/code> with flame graphs and live view.<\/li>\n<li><b>Also new:<\/b> <code>sentinel()<\/code> built-in, TypedDict <code>extra_items<\/code>, smarter error messages and a faster JIT.<\/li>\n<\/ul>\n<\/div>\n\n<nav class=\"dd-toc\" aria-label=\"Table of contents\">\n<strong>Table of contents<\/strong>\n<ol>\n<li><a href=\"#release-timeline\">Python 3.15 release timeline<\/a><\/li>\n<li><a href=\"#install\">How to try Python 3.15 today<\/a><\/li>\n<li><a href=\"#lazy-imports\">Lazy imports (PEP 810)<\/a><\/li>\n<li><a href=\"#frozendict\">frozendict built-in (PEP 814)<\/a><\/li>\n<li><a href=\"#comprehension-unpacking\">Unpacking in comprehensions<\/a><\/li>\n<li><a href=\"#utf8-default\">UTF-8 is the default<\/a><\/li>\n<li><a href=\"#tachyon-profiler\">Tachyon sampling profiler<\/a><\/li>\n<li><a href=\"#sentinel-typing\">sentinel &amp; typing upgrades<\/a><\/li>\n<li><a href=\"#error-messages\">Better error messages &amp; JIT<\/a><\/li>\n<li><a href=\"#common-errors\">Common errors &amp; fixes<\/a><\/li>\n<li><a href=\"#best-practices\">Upgrade best practices<\/a><\/li>\n<li><a href=\"#faq\">FAQ<\/a><\/li>\n<\/ol>\n<\/nav>\n<\/div>\n\n\n\n<div class=\"dd-post\">\n<h2 id=\"release-timeline\">Python 3.15 Release Timeline: When Can You Use It?<\/h2>\n<p>Python follows a yearly release cycle (<a href=\"https:\/\/peps.python.org\/pep-0602\/\" target=\"_blank\" rel=\"noopener\">PEP 602<\/a>): roughly 17 months of development and a new feature version every October. According to the official release schedule in <a href=\"https:\/\/peps.python.org\/pep-0790\/\" target=\"_blank\" rel=\"noopener\">PEP 790<\/a>, Python 3.15 went through eight alphas, four betas and three release candidates before the final release.<\/p>\n\n<figure>\n<svg viewBox=\"0 0 1000 260\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" role=\"img\" aria-labelledby=\"ddTlT ddTlD\">\n<title id=\"ddTlT\">Python 3.15 release timeline<\/title>\n<desc id=\"ddTlD\">Timeline: development started May 2025, alphas from October 2025 to April 2026, betas May to July 2026, release candidates August to early October 2026, final release October 9, 2026, then bugfix updates for two years and security fixes until about October 2031.<\/desc>\n<rect width=\"1000\" height=\"260\" rx=\"14\" fill=\"#0f1430\"\/>\n<line x1=\"60\" y1=\"130\" x2=\"940\" y2=\"130\" stroke=\"#2f3a7a\" stroke-width=\"6\" stroke-linecap=\"round\"\/>\n<line x1=\"60\" y1=\"130\" x2=\"800\" y2=\"130\" stroke=\"#3178c6\" stroke-width=\"6\" stroke-linecap=\"round\"\/>\n<g font-family=\"Segoe UI, Roboto, Arial, sans-serif\" text-anchor=\"middle\">\n<circle cx=\"80\" cy=\"130\" r=\"12\" fill=\"#3178c6\"\/><text x=\"80\" y=\"95\" fill=\"#fff\" font-size=\"16\" font-weight=\"700\">Dev starts<\/text><text x=\"80\" y=\"172\" fill=\"#aab4ff\" font-size=\"14\">May 2025<\/text>\n<circle cx=\"270\" cy=\"130\" r=\"12\" fill=\"#3178c6\"\/><text x=\"270\" y=\"95\" fill=\"#fff\" font-size=\"16\" font-weight=\"700\">Alpha 1\u20138<\/text><text x=\"270\" y=\"172\" fill=\"#aab4ff\" font-size=\"14\">Oct 2025 \u2013 Apr 2026<\/text>\n<circle cx=\"460\" cy=\"130\" r=\"12\" fill=\"#3178c6\"\/><text x=\"460\" y=\"95\" fill=\"#fff\" font-size=\"16\" font-weight=\"700\">Beta 1\u20134<\/text><text x=\"460\" y=\"172\" fill=\"#aab4ff\" font-size=\"14\">May \u2013 Jul 2026<\/text><text x=\"460\" y=\"194\" fill=\"#6b76a8\" font-size=\"13\">feature freeze<\/text>\n<circle cx=\"640\" cy=\"130\" r=\"12\" fill=\"#3178c6\"\/><text x=\"640\" y=\"95\" fill=\"#fff\" font-size=\"16\" font-weight=\"700\">RC 1\u20133<\/text><text x=\"640\" y=\"172\" fill=\"#aab4ff\" font-size=\"14\">Aug \u2013 Oct 2, 2026<\/text>\n<circle cx=\"800\" cy=\"130\" r=\"16\" fill=\"#7c5cff\"><animate attributeName=\"r\" values=\"14;19;14\" dur=\"2s\" repeatCount=\"indefinite\"\/><\/circle><text x=\"800\" y=\"90\" fill=\"#fff\" font-size=\"18\" font-weight=\"800\">3.15.0 final<\/text><text x=\"800\" y=\"172\" fill=\"#c9bcff\" font-size=\"15\" font-weight=\"700\">Oct 9, 2026<\/text>\n<circle cx=\"925\" cy=\"130\" r=\"10\" fill=\"#2f3a7a\" stroke=\"#6b76a8\"\/><text x=\"925\" y=\"95\" fill=\"#c3c9e8\" font-size=\"14\">Support<\/text><text x=\"925\" y=\"172\" fill=\"#6b76a8\" font-size=\"13\">to ~Oct 2031<\/text>\n<\/g>\n<\/svg>\n<figcaption>Figure 1: The Python 3.15 release timeline (source: <a href=\"https:\/\/peps.python.org\/pep-0790\/\" target=\"_blank\" rel=\"noopener\">PEP 790<\/a>). Bugfix releases come for about two years, then security-only fixes until around October 2031.<\/figcaption>\n<\/figure>\n\n<div class=\"dd-note\"><b>Note:<\/b> If you are reading this before October 9, 2026, use the release candidate for testing only. The release candidates are feature-complete, so everything in this article will be in the final version.<\/div>\n\n<h2 id=\"install\">How to Try Python 3.15 Today<\/h2>\n<p>You do not need to replace your system Python. Pick one of these isolated options. The CSS tabs below switch between them (no JavaScript involved).<\/p>\n\n<div class=\"dd-tabs\">\n<input type=\"radio\" name=\"dd-install\" id=\"dd-t1\" checked>\n<input type=\"radio\" name=\"dd-install\" id=\"dd-t2\">\n<input type=\"radio\" name=\"dd-install\" id=\"dd-t3\">\n<input type=\"radio\" name=\"dd-install\" id=\"dd-t4\">\n<div class=\"dd-tab-labels\">\n<label for=\"dd-t1\">uv<\/label>\n<label for=\"dd-t2\">Docker<\/label>\n<label for=\"dd-t3\">pyenv<\/label>\n<label for=\"dd-t4\">Windows \/ macOS<\/label>\n<\/div>\n<div class=\"dd-panel dd-p1\">\n<pre><code><span class=\"tok-c\"># Install uv first: https:\/\/docs.astral.sh\/uv\/<\/span>\nuv self update\nuv python install 3.15\nuv venv --python 3.15 .venv315\nsource .venv315\/bin\/activate      <span class=\"tok-c\"># Windows: .venv315\\Scripts\\activate<\/span>\npython --version<\/code><\/pre>\n<p>Fastest option. If only a release candidate is available, ask for it explicitly, e.g. <code>uv python install 3.15.0rc3<\/code>.<\/p>\n<\/div>\n<div class=\"dd-panel dd-p2\">\n<pre><code><span class=\"tok-c\"># Before the final release use the rc tag; afterwards use python:3.15-slim<\/span>\ndocker run --rm -it python:3.15-rc-slim python\n\n<span class=\"tok-c\"># Run your project's tests in a throwaway container<\/span>\ndocker run --rm -v \"$PWD\":\/app -w \/app python:3.15-rc-slim \\\n  sh -c \"pip install -r requirements.txt &amp;&amp; python -m pytest\"<\/code><\/pre>\n<p>Best for CI and for checking that your dependencies install cleanly.<\/p>\n<\/div>\n<div class=\"dd-panel dd-p3\">\n<pre><code>pyenv update\npyenv install --list | grep \"3.15\"\npyenv install 3.15.0       <span class=\"tok-c\"># or the latest 3.15.0rcN<\/span>\npyenv local 3.15.0\npython --version<\/code><\/pre>\n<p>Good if you already manage multiple versions with pyenv (or pyenv-win on Windows).<\/p>\n<\/div>\n<div class=\"dd-panel dd-p4\">\n<pre><code><span class=\"tok-c\"># Download the installer from python.org\/downloads<\/span>\n<span class=\"tok-c\"># Windows (Python install manager):<\/span>\npy install 3.15\npy -3.15 --version<\/code><\/pre>\n<p>Official installers from python.org. On Windows, the 64-bit build now uses the faster tail-calling interpreter, and the macOS installer includes a free-threaded build.<\/p>\n<\/div>\n<\/div>\n<\/div>\n\n\n\n<div class=\"dd-post\">\n<h2 id=\"lazy-imports\">1. Lazy Imports in Python 3.15 (<a href=\"https:\/\/peps.python.org\/pep-0810\/\" target=\"_blank\" rel=\"noopener\">PEP 810<\/a>)<\/h2>\n<p>This is the headline feature. Large applications often spend hundreds of milliseconds at startup just importing modules they may never use in that run \u2014 think of a CLI where <code>--help<\/code> loads pandas, boto3 and your whole ORM. With <a href=\"https:\/\/peps.python.org\/pep-0810\/\" target=\"_blank\" rel=\"noopener\">PEP 810<\/a>, you can mark an import as <strong>lazy<\/strong>. Python creates a lightweight placeholder, and the real import happens only when the name is first used.<\/p>\n\n<span class=\"dd-file\">cli.py<\/span>\n<pre><code><span class=\"tok-k\">lazy import<\/span> json\n<span class=\"tok-k\">lazy from<\/span> pathlib <span class=\"tok-k\">import<\/span> Path\n<span class=\"tok-k\">lazy import<\/span> csv\n\n<span class=\"tok-k\">import<\/span> sys   <span class=\"tok-c\"># normal (eager) import still works<\/span>\n\n\n<span class=\"tok-k\">def<\/span> <span class=\"tok-f\">export<\/span>(rows, out):\n    <span class=\"tok-c\"># csv is actually imported here, on first use<\/span>\n    <span class=\"tok-k\">with<\/span> Path(out).open(<span class=\"tok-s\">\"w\"<\/span>, newline=<span class=\"tok-s\">\"\"<\/span>) <span class=\"tok-k\">as<\/span> f:\n        csv.writer(f).writerows(rows)\n\n\n<span class=\"tok-k\">if<\/span> __name__ == <span class=\"tok-s\">\"__main__\"<\/span>:\n    <span class=\"tok-k\">if<\/span> <span class=\"tok-s\">\"--help\"<\/span> <span class=\"tok-k\">in<\/span> sys.argv:\n        print(<span class=\"tok-s\">\"usage: cli.py data.json out.csv\"<\/span>)   <span class=\"tok-c\"># json\/csv never loaded<\/span>\n        sys.exit(<span class=\"tok-n\">0<\/span>)\n    data = json.loads(Path(sys.argv[<span class=\"tok-n\">1<\/span>]).read_text())\n    export(data, sys.argv[<span class=\"tok-n\">2<\/span>])<\/code><\/pre>\n\n<figure>\n<svg viewBox=\"0 0 1000 300\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" role=\"img\" aria-labelledby=\"ddLzT ddLzD\">\n<title id=\"ddLzT\">Eager vs lazy import startup<\/title>\n<desc id=\"ddLzD\">Comparison of two startup timelines. Eager: all modules are imported before main runs, so startup is long. Lazy: only a placeholder is created at startup, main starts quickly, and each module loads only when first used.<\/desc>\n<rect width=\"1000\" height=\"300\" rx=\"14\" fill=\"#0f1430\"\/>\n<g font-family=\"Segoe UI, Roboto, Arial, sans-serif\">\n<text x=\"30\" y=\"52\" fill=\"#fff\" font-size=\"18\" font-weight=\"700\">Eager import<\/text>\n<rect x=\"190\" y=\"30\" width=\"150\" height=\"34\" rx=\"6\" fill=\"#3178c6\"\/><text x=\"265\" y=\"53\" fill=\"#fff\" font-size=\"14\" text-anchor=\"middle\">import pandas<\/text>\n<rect x=\"344\" y=\"30\" width=\"110\" height=\"34\" rx=\"6\" fill=\"#3178c6\"\/><text x=\"399\" y=\"53\" fill=\"#fff\" font-size=\"14\" text-anchor=\"middle\">import boto3<\/text>\n<rect x=\"458\" y=\"30\" width=\"90\" height=\"34\" rx=\"6\" fill=\"#3178c6\"\/><text x=\"503\" y=\"53\" fill=\"#fff\" font-size=\"14\" text-anchor=\"middle\">import orm<\/text>\n<rect x=\"552\" y=\"30\" width=\"160\" height=\"34\" rx=\"6\" fill=\"#22c55e\"\/><text x=\"632\" y=\"53\" fill=\"#0b1020\" font-size=\"14\" font-weight=\"700\" text-anchor=\"middle\">main() runs<\/text>\n<text x=\"190\" y=\"90\" fill=\"#aab4ff\" font-size=\"13\">startup cost paid up front, even for &#8211;help<\/text>\n\n<text x=\"30\" y=\"172\" fill=\"#fff\" font-size=\"18\" font-weight=\"700\">Lazy import<\/text>\n<rect x=\"190\" y=\"150\" width=\"60\" height=\"34\" rx=\"6\" fill=\"#2f3a7a\" stroke=\"#7c5cff\"\/><text x=\"220\" y=\"172\" fill=\"#cfd6ff\" font-size=\"12\" text-anchor=\"middle\">proxies<\/text>\n<rect x=\"254\" y=\"150\" width=\"160\" height=\"34\" rx=\"6\" fill=\"#22c55e\"\/><text x=\"334\" y=\"173\" fill=\"#0b1020\" font-size=\"14\" font-weight=\"700\" text-anchor=\"middle\">main() runs<\/text>\n<rect x=\"418\" y=\"150\" width=\"150\" height=\"34\" rx=\"6\" fill=\"#7c5cff\"\/><text x=\"493\" y=\"173\" fill=\"#fff\" font-size=\"14\" text-anchor=\"middle\">pandas (on use)<\/text>\n<text x=\"190\" y=\"210\" fill=\"#aab4ff\" font-size=\"13\">boto3 and orm are never loaded if this run doesn&#8217;t touch them<\/text>\n<line x1=\"190\" y1=\"250\" x2=\"940\" y2=\"250\" stroke=\"#3a4690\" stroke-width=\"2\"\/>\n<text x=\"940\" y=\"275\" fill=\"#6b76a8\" font-size=\"13\" text-anchor=\"end\">time \u2192<\/text>\n<circle r=\"6\" fill=\"#c9bcff\"><animateMotion dur=\"4s\" repeatCount=\"indefinite\" path=\"M190 250 L940 250\"\/><\/circle>\n<\/g>\n<\/svg>\n<figcaption>Figure 2: With lazy imports, your program reaches <code>main()<\/code> sooner and only pays for the modules it actually uses.<\/figcaption>\n<\/figure>\n\n<h3>Rules you must know about lazy imports<\/h3>\n<ul>\n<li><strong>Module level only.<\/strong> <code>lazy import<\/code> inside a function, a class body or a <code>try\/except<\/code> block is a <code>SyntaxError<\/code>.<\/li>\n<li><strong>No star or future imports.<\/strong> <code>lazy from x import *<\/code> and <code>lazy from __future__ import ...<\/code> are not allowed.<\/li>\n<li><strong>Errors move to first use.<\/strong> A typo like <code>lazy from json import dumsp<\/code> does not fail at import time; it fails when you call <code>dumsp()<\/code>. The traceback shows both lines so it is still easy to debug.<\/li>\n<li><strong>Side-effect imports.<\/strong> If a module registers plugins or patches things when imported, keep it eager \u2014 otherwise those side effects happen later than you expect.<\/li>\n<\/ul>\n\n<h3>Supporting Python 3.14 and 3.15 at the same time<\/h3>\n<p>The <code>lazy<\/code> keyword is a syntax error on older Pythons. For libraries that must support several versions, <a href=\"https:\/\/peps.python.org\/pep-0810\/\" target=\"_blank\" rel=\"noopener\">PEP 810<\/a> adds <code>__lazy_modules__<\/code>. Older interpreters simply ignore it, so the imports stay eager there:<\/p>\n<pre><code><span class=\"tok-c\"># Lazy on 3.15+, normal eager imports on 3.14 and below<\/span>\n__lazy_modules__ = [<span class=\"tok-s\">\"pandas\"<\/span>, <span class=\"tok-s\">\"boto3\"<\/span>]\n\n<span class=\"tok-k\">import<\/span> pandas <span class=\"tok-k\">as<\/span> pd\n<span class=\"tok-k\">import<\/span> boto3<\/code><\/pre>\n\n<h3>Global switches and measuring the win<\/h3>\n<pre><code><span class=\"tok-c\"># Measure import time before and after (works on older versions too)<\/span>\npython -X importtime -c \"import myapp\" 2&gt; import.log\n\n<span class=\"tok-c\"># Experiment: make ALL imports lazy for this run<\/span>\npython -X lazy_imports=all -m myapp\nPYTHON_LAZY_IMPORTS=all python -m myapp\n\n<span class=\"tok-c\"># Inside Python<\/span>\npython -c \"import sys; print(sys.get_lazy_imports())\"   <span class=\"tok-c\"># 'normal' by default<\/span><\/code><\/pre>\n<div class=\"dd-tip\"><b>Tip:<\/b> Try <code>-X lazy_imports=all<\/code> on your test suite first. If everything passes, you have a free startup boost; if something breaks, you have found an import with hidden side effects.<\/div>\n<\/div>\n\n\n\n<div class=\"dd-post\">\n<h2 id=\"frozendict\">2. frozendict: A Built-in Immutable Dictionary (<a href=\"https:\/\/peps.python.org\/pep-0814\/\" target=\"_blank\" rel=\"noopener\">PEP 814<\/a>)<\/h2>\n<p>Python has had <code>frozenset<\/code> for years, but no frozen dict. Developers used <code>MappingProxyType<\/code>, tuples of pairs, or third-party packages. Python 3.15 adds <code>frozendict<\/code> as a built-in. It cannot be changed after creation and \u2014 because it is immutable \u2014 it is <strong>hashable<\/strong>, so you can use it as a dict key, put it in a set, or pass it to <code>functools.lru_cache<\/code>.<\/p>\n<pre><code>&gt;&gt;&gt; cfg = frozendict(host=<span class=\"tok-s\">\"localhost\"<\/span>, port=<span class=\"tok-n\">5432<\/span>)\n&gt;&gt;&gt; cfg\nfrozendict({'host': 'localhost', 'port': 5432})\n&gt;&gt;&gt; cfg[<span class=\"tok-s\">\"port\"<\/span>] = <span class=\"tok-n\">6543<\/span>\nTypeError: 'frozendict' object does not support item assignment\n&gt;&gt;&gt; prod = cfg | {<span class=\"tok-s\">\"host\"<\/span>: <span class=\"tok-s\">\"db.internal\"<\/span>}   <span class=\"tok-c\"># merge returns a NEW frozendict<\/span>\n&gt;&gt;&gt; hash(frozendict(a=<span class=\"tok-n\">1<\/span>, b=<span class=\"tok-n\">2<\/span>)) == hash(frozendict(b=<span class=\"tok-n\">2<\/span>, a=<span class=\"tok-n\">1<\/span>))\nTrue<\/code><\/pre>\n\n<h3>Real-world use: caching an expensive lookup<\/h3>\n<span class=\"dd-file\">pricing.py<\/span>\n<pre><code><span class=\"tok-k\">from<\/span> functools <span class=\"tok-k\">import<\/span> lru_cache\n\n<span class=\"tok-c\"># A dict argument would raise \"TypeError: unhashable type: 'dict'\"<\/span>\n<span class=\"tok-f\">@lru_cache<\/span>(maxsize=<span class=\"tok-n\">1024<\/span>)\n<span class=\"tok-k\">def<\/span> <span class=\"tok-f\">quote<\/span>(filters: frozendict) -&gt; float:\n    print(<span class=\"tok-s\">\"computing...\"<\/span>)\n    base = <span class=\"tok-n\">100.0<\/span>\n    <span class=\"tok-k\">if<\/span> filters.get(<span class=\"tok-s\">\"region\"<\/span>) == <span class=\"tok-s\">\"IN\"<\/span>:\n        base *= <span class=\"tok-n\">0.8<\/span>\n    <span class=\"tok-k\">return<\/span> base * filters.get(<span class=\"tok-s\">\"qty\"<\/span>, <span class=\"tok-n\">1<\/span>)\n\n\nprint(quote(frozendict(region=<span class=\"tok-s\">\"IN\"<\/span>, qty=<span class=\"tok-n\">3<\/span>)))   <span class=\"tok-c\"># computing... 240.0<\/span>\nprint(quote(frozendict(qty=<span class=\"tok-n\">3<\/span>, region=<span class=\"tok-s\">\"IN\"<\/span>)))   <span class=\"tok-c\"># cache hit: 240.0<\/span><\/code><\/pre>\n<p>When you accept &#8220;any mapping&#8221; in a function, check with <code>isinstance(x, collections.abc.Mapping)<\/code> or <code>isinstance(x, (dict, frozendict))<\/code>. Note that <code>frozendict<\/code> is <em>not<\/em> a subclass of <code>dict<\/code>, so <code>isinstance(x, dict)<\/code> returns <code>False<\/code>. The standard library modules <code>json<\/code>, <code>pickle<\/code>, <code>copy<\/code> and <code>pprint<\/code> already understand it.<\/p>\n\n<h2 id=\"comprehension-unpacking\">3. Unpacking in Comprehensions (<a href=\"https:\/\/peps.python.org\/pep-0798\/\" target=\"_blank\" rel=\"noopener\">PEP 798<\/a>)<\/h2>\n<p>Flattening a list of lists used to need a double loop that many beginners find confusing: <code>[x for row in rows for x in row]<\/code>. Python 3.15 lets you use <code>*<\/code> and <code>**<\/code> directly inside comprehensions:<\/p>\n<pre><code>&gt;&gt;&gt; rows = [[<span class=\"tok-n\">1<\/span>, <span class=\"tok-n\">2<\/span>], [<span class=\"tok-n\">3<\/span>, <span class=\"tok-n\">4<\/span>], [<span class=\"tok-n\">5<\/span>]]\n&gt;&gt;&gt; [*row <span class=\"tok-k\">for<\/span> row <span class=\"tok-k\">in<\/span> rows]\n[1, 2, 3, 4, 5]\n\n&gt;&gt;&gt; tag_sets = [{<span class=\"tok-s\">\"py\"<\/span>, <span class=\"tok-s\">\"api\"<\/span>}, {<span class=\"tok-s\">\"api\"<\/span>, <span class=\"tok-s\">\"db\"<\/span>}]\n&gt;&gt;&gt; {*s <span class=\"tok-k\">for<\/span> s <span class=\"tok-k\">in<\/span> tag_sets}\n{'py', 'api', 'db'}\n\n&gt;&gt;&gt; layers = [{<span class=\"tok-s\">\"debug\"<\/span>: False, <span class=\"tok-s\">\"port\"<\/span>: <span class=\"tok-n\">80<\/span>}, {<span class=\"tok-s\">\"debug\"<\/span>: True}]\n&gt;&gt;&gt; {**layer <span class=\"tok-k\">for<\/span> layer <span class=\"tok-k\">in<\/span> layers}       <span class=\"tok-c\"># later dicts win<\/span>\n{'debug': True, 'port': 80}\n\n&gt;&gt;&gt; gen = (*row <span class=\"tok-k\">for<\/span> row <span class=\"tok-k\">in<\/span> rows)          <span class=\"tok-c\"># works in generators too<\/span>\n&gt;&gt;&gt; sum(gen)\n15<\/code><\/pre>\n<p>A nice backend example: merging config layers (defaults \u2192 environment \u2192 CLI flags) is now a single readable expression, <code>settings = {**layer for layer in (defaults, env_cfg, cli_cfg)}<\/code>.<\/p>\n\n<h2 id=\"utf8-default\">4. UTF-8 Is Now the Default Encoding (<a href=\"https:\/\/peps.python.org\/pep-0686\/\" target=\"_blank\" rel=\"noopener\">PEP 686<\/a>)<\/h2>\n<p>Before 3.15, <code>open(\"file.txt\")<\/code> used your <em>locale<\/em> encoding. On Linux and macOS that was usually UTF-8, but on many Windows machines in India and elsewhere it was <code>cp1252<\/code>. Code that worked on your Mac crashed on a teammate&#8217;s Windows laptop with <code>UnicodeDecodeError<\/code> as soon as a file contained &#8220;\u20b9&#8221;, &#8220;\u0928\u092e\u0938\u094d\u0924\u0947&#8221; or an emoji. Python 3.15 turns on UTF-8 Mode by default, so text I\/O uses UTF-8 on every platform.<\/p>\n<pre><code><span class=\"tok-c\"># Python 3.15: UTF-8 on every OS<\/span>\n<span class=\"tok-k\">with<\/span> open(<span class=\"tok-s\">\"invoice.txt\"<\/span>, <span class=\"tok-s\">\"w\"<\/span>) <span class=\"tok-k\">as<\/span> f:\n    f.write(<span class=\"tok-s\">\"Total: \u20b91,499\"<\/span>)\n\n<span class=\"tok-c\"># Need the old behaviour for a legacy file? Ask for it explicitly:<\/span>\n<span class=\"tok-k\">with<\/span> open(<span class=\"tok-s\">\"old_export.csv\"<\/span>, encoding=<span class=\"tok-s\">\"locale\"<\/span>) <span class=\"tok-k\">as<\/span> f:\n    data = f.read()<\/code><\/pre>\n<pre><code><span class=\"tok-c\"># Turn UTF-8 Mode off for a whole process (temporary escape hatch)<\/span>\npython -X utf8=0 legacy_script.py\nPYTHONUTF8=0 python legacy_script.py<\/code><\/pre>\n<div class=\"dd-tip\"><b>Best practice:<\/b> Keep writing <code>encoding=\"utf-8\"<\/code> explicitly in libraries that still support older Pythons. It is clear, and it behaves the same on every version.<\/div>\n<\/div>\n\n\n\n<div class=\"dd-post\">\n<h2 id=\"tachyon-profiler\">5. Tachyon: The New Sampling Profiler (<a href=\"https:\/\/peps.python.org\/pep-0799\/\" target=\"_blank\" rel=\"noopener\">PEP 799<\/a>)<\/h2>\n<p>Python 3.15 reorganises profiling into a new <code>profiling<\/code> package. <code>profiling.tracing<\/code> is the deterministic profiler you know from <code>cProfile<\/code> (which stays as an alias), and <code>profiling.sampling<\/code> is <strong>Tachyon<\/strong> \u2014 a high-frequency <em>statistical<\/em> profiler. Instead of instrumenting every function call, it samples the call stack many times per second, so the overhead is very low and you can even attach it to a process that is already running in production.<\/p>\n<pre><code><span class=\"tok-c\"># Profile a script and open an interactive HTML flame graph<\/span>\npython -m profiling.sampling run --flamegraph app.py\n\n<span class=\"tok-c\"># Profile a module (e.g. a FastAPI app started via uvicorn)<\/span>\npython -m profiling.sampling run --live -m uvicorn main:app\n\n<span class=\"tok-c\"># Attach to an already-running process by PID, all threads<\/span>\npython -m profiling.sampling attach -a 12345\n\n<span class=\"tok-c\"># Only count CPU time (ignore time spent waiting on I\/O)<\/span>\npython -m profiling.sampling run --mode cpu --pstats app.py\n\n<span class=\"tok-c\"># Where is my asyncio code stuck? Dump stacks with task info<\/span>\npython -m profiling.sampling dump --async-aware 12345<\/code><\/pre>\n\n<div class=\"dd-cards\">\n<div class=\"dd-card\"><h4>&#8211;flamegraph<\/h4><p>Interactive HTML flame graph \u2014 the quickest way to spot hot paths.<\/p><\/div>\n<div class=\"dd-card\"><h4>&#8211;heatmap<\/h4><p>Line-level HTML heatmap of your source files.<\/p><\/div>\n<div class=\"dd-card\"><h4>&#8211;live<\/h4><p>A <code>top<\/code>-like terminal view that updates in real time.<\/p><\/div>\n<div class=\"dd-card\"><h4>&#8211;gecko \/ &#8211;collapsed<\/h4><p>Export to Firefox Profiler or any flame-graph tool.<\/p><\/div>\n<\/div>\n<p>Modes matter: <code>--mode wall<\/code> (default) measures real elapsed time and is ideal for slow API endpoints that wait on databases; <code>--mode cpu<\/code> shows pure computation; <code>--mode gil<\/code> shows who is holding the GIL. The old pure-Python <code>profile<\/code> module is deprecated and scheduled for removal in Python 3.17.<\/p>\n\n<h2 id=\"sentinel-typing\">6. sentinel() Built-in and Typing Upgrades<\/h2>\n<h3>sentinel (<a href=\"https:\/\/peps.python.org\/pep-0661\/\" target=\"_blank\" rel=\"noopener\">PEP 661<\/a>)<\/h3>\n<p>Ever needed to tell the difference between &#8220;argument not passed&#8221; and &#8220;argument passed as <code>None<\/code>&#8220;? People used <code>object()<\/code>, which has an ugly repr and does not work nicely with type checkers. Python 3.15 adds a <code>sentinel<\/code> built-in:<\/p>\n<span class=\"dd-file\">repo.py<\/span>\n<pre><code>MISSING = sentinel(<span class=\"tok-s\">\"MISSING\"<\/span>)\n\n<span class=\"tok-k\">def<\/span> <span class=\"tok-f\">update_user<\/span>(user_id: int, *, email: str | None | MISSING = MISSING) -&gt; dict:\n    changes = {}\n    <span class=\"tok-k\">if<\/span> email <span class=\"tok-k\">is not<\/span> MISSING:\n        changes[<span class=\"tok-s\">\"email\"<\/span>] = email      <span class=\"tok-c\"># None means \"clear the email\"<\/span>\n    <span class=\"tok-k\">return<\/span> changes\n\nprint(update_user(<span class=\"tok-n\">1<\/span>))               <span class=\"tok-c\"># {}<\/span>\nprint(update_user(<span class=\"tok-n\">1<\/span>, email=None))   <span class=\"tok-c\"># {'email': None}<\/span>\nprint(MISSING)                      <span class=\"tok-c\"># MISSING  (a readable repr)<\/span><\/code><\/pre>\n<p>Sentinels compare equal only to themselves (always use <code>is<\/code>), survive <code>copy<\/code>\/<code>deepcopy<\/code>, can be pickled when defined at module level, and support <code>|<\/code> in type hints.<\/p>\n\n<h3>TypedDict with extra_items and closed (<a href=\"https:\/\/peps.python.org\/pep-0728\/\" target=\"_blank\" rel=\"noopener\">PEP 728<\/a>)<\/h3>\n<p>Very handy for typing JSON payloads in APIs. You can now say what type <em>extra<\/em> keys must have, or forbid them entirely:<\/p>\n<pre><code><span class=\"tok-k\">from<\/span> typing <span class=\"tok-k\">import<\/span> TypedDict\n\n<span class=\"tok-k\">class<\/span> <span class=\"tok-f\">Movie<\/span>(TypedDict, extra_items=bool):\n    name: str          <span class=\"tok-c\"># any other key must be a bool<\/span>\n\n<span class=\"tok-k\">class<\/span> <span class=\"tok-f\">StrictUser<\/span>(TypedDict, closed=True):\n    id: int\n    email: str         <span class=\"tok-c\"># no extra keys allowed at all<\/span>\n\nm: Movie = {<span class=\"tok-s\">\"name\"<\/span>: <span class=\"tok-s\">\"Dangal\"<\/span>, <span class=\"tok-s\">\"is_hit\"<\/span>: True}   <span class=\"tok-c\"># OK for type checkers<\/span><\/code><\/pre>\n<p>Python 3.15 also adds <code>TypeForm<\/code> (<a href=\"https:\/\/peps.python.org\/pep-0747\/\" target=\"_blank\" rel=\"noopener\">PEP 747<\/a>) for functions that accept a type expression as a value \u2014 useful for validation libraries like Pydantic-style parsers.<\/p>\n\n<h2 id=\"error-messages\">7. Smarter Error Messages and a Faster JIT<\/h2>\n<p>Python keeps getting friendlier for people coming from JavaScript or Java. The interpreter now recognises common method names from other languages:<\/p>\n<pre><code>&gt;&gt;&gt; [<span class=\"tok-n\">1<\/span>, <span class=\"tok-n\">2<\/span>, <span class=\"tok-n\">3<\/span>].push(<span class=\"tok-n\">4<\/span>)\nAttributeError: 'list' object has no attribute 'push'. Did you mean '.append'?\n&gt;&gt;&gt; <span class=\"tok-s\">\"hello\"<\/span>.toUpperCase()\nAttributeError: 'str' object has no attribute 'toUpperCase'. Did you mean '.upper'?\n&gt;&gt;&gt; {}.put(<span class=\"tok-s\">\"a\"<\/span>, <span class=\"tok-n\">1<\/span>)\nAttributeError: 'dict' object has no attribute 'put'. Use d[k] = v.\n&gt;&gt;&gt; (<span class=\"tok-n\">1<\/span>, <span class=\"tok-n\">2<\/span>).append(<span class=\"tok-n\">3<\/span>)\nAttributeError: 'tuple' object has no attribute 'append'. Did you mean to use a 'list' object?<\/code><\/pre>\n<p>It can even suggest nested attributes, e.g. <em>&#8220;Did you mean &#8216;.inner.area&#8217; instead of &#8216;.area&#8217;?&#8221;<\/em>. On performance, the experimental JIT compiler got a big upgrade: the release notes report around an 8\u20139% geometric-mean speedup on x86-64 Linux and 12\u201313% on Apple Silicon macOS compared to the standard interpreter. Frame pointers are also enabled by default (<a href=\"https:\/\/peps.python.org\/pep-0831\/\" target=\"_blank\" rel=\"noopener\">PEP 831<\/a>), which makes system profilers like <code>perf<\/code> much more useful.<\/p>\n<p>If you are building AI tooling in Python or TypeScript, these startup and profiling gains matter for servers that start often \u2014 see our guide on <a href=\"https:\/\/devdojo.co.in\/build-mcp-server-typescript\/\">how to build an MCP server<\/a> for a related example of a short-lived developer tool process.<\/p>\n<\/div>\n\n\n\n<div class=\"dd-post\">\n<h2 id=\"common-errors\">Common Python 3.15 Upgrade Errors &amp; Fixes<\/h2>\n<div class=\"dd-table-wrap\">\n<table>\n<thead><tr><th>Error \/ symptom<\/th><th>Why it happens<\/th><th>Fix<\/th><\/tr><\/thead>\n<tbody>\n<tr><td><code>SyntaxError<\/code> on <code>lazy import<\/code><\/td><td>Used inside a function, class or <code>try<\/code> block \u2014 or running on Python \u2264 3.14.<\/td><td>Move it to module level; for older versions use <code>__lazy_modules__<\/code> instead.<\/td><\/tr>\n<tr><td><code>ImportError<\/code> appears &#8220;late&#8221;, deep in a request<\/td><td>Lazy imports raise at first use, not at startup.<\/td><td>Add a smoke test that touches each lazy name, or keep critical imports eager.<\/td><\/tr>\n<tr><td>Plugin \/ signal handler silently not registered<\/td><td>A module with import-time side effects was made lazy.<\/td><td>Import such modules eagerly (no <code>lazy<\/code>, not in <code>__lazy_modules__<\/code>).<\/td><\/tr>\n<tr><td><code>UnicodeDecodeError<\/code> reading an old Windows file<\/td><td>UTF-8 is now the default; the file was saved in cp1252.<\/td><td><code>open(path, encoding=\"locale\")<\/code> or the real encoding, e.g. <code>\"cp1252\"<\/code>.<\/td><\/tr>\n<tr><td><code>TypeError: 'frozendict' object does not support item assignment<\/code><\/td><td>Trying to mutate an immutable mapping.<\/td><td>Create a new one: <code>new = fd | {\"k\": v}<\/code>, or convert with <code>dict(fd)<\/code>.<\/td><\/tr>\n<tr><td><code>isinstance(x, dict)<\/code> is False for frozendict<\/td><td><code>frozendict<\/code> is not a <code>dict<\/code> subclass.<\/td><td>Check <code>collections.abc.Mapping<\/code> instead.<\/td><\/tr>\n<tr><td><code>DeprecationWarning<\/code> for <code>profile<\/code><\/td><td>The pure-Python profiler is deprecated (removal in 3.17).<\/td><td>Switch to <code>profiling.tracing<\/code> \/ <code>cProfile<\/code> or Tachyon.<\/td><\/tr>\n<tr><td><code>pip install<\/code> builds from source and fails<\/td><td>A C-extension package has no 3.15 wheels yet.<\/td><td>Upgrade the package, wait for wheels, or keep production on 3.14 for now.<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n\n<h2 id=\"best-practices\">Python 3.15 Upgrade Best Practices<\/h2>\n<ol>\n<li><strong>Test in CI first.<\/strong> Add <code>\"3.15\"<\/code> to your GitHub Actions matrix (or a <code>python:3.15-slim<\/code> Docker job) and run the full test suite with <code>-W error::DeprecationWarning<\/code>.<\/li>\n<li><strong>Check your dependencies.<\/strong> Big libraries such as NumPy, Pydantic and database drivers usually publish wheels within days or weeks of the release. Run <code>pip install -r requirements.txt<\/code> on 3.15 early so you know which ones block you.<\/li>\n<li><strong>Adopt lazy imports where startup matters.<\/strong> CLIs, serverless functions (AWS Lambda, Cloud Run cold starts) and dev tools benefit most. Long-running web servers benefit less, since imports happen only once.<\/li>\n<li><strong>Be explicit about encodings at boundaries.<\/strong> When reading files produced by other systems (Excel exports, bank CSVs), specify the encoding instead of relying on any default.<\/li>\n<li><strong>Use frozendict for config and cache keys.<\/strong> Immutable settings objects prevent a whole class of &#8220;who changed this at runtime?&#8221; bugs.<\/li>\n<li><strong>Profile before optimising.<\/strong> Run Tachyon with <code>--flamegraph<\/code> on a slow endpoint before rewriting anything.<\/li>\n<li><strong>Roll out gradually.<\/strong> Many teams wait for 3.15.1 or 3.15.2 for critical production services \u2014 a reasonable, low-risk approach.<\/li>\n<\/ol>\n<span class=\"dd-file\">.github\/workflows\/test.yml<\/span>\n<pre><code>jobs:\n  test:\n    runs-on: ubuntu-latest\n    strategy:\n      matrix:\n        python-version: [\"3.13\", \"3.14\", \"3.15\"]\n    steps:\n      - uses: actions\/checkout@v5\n      - uses: actions\/setup-python@v6\n        with:\n          python-version: ${{ matrix.python-version }}\n          allow-prereleases: true\n      - run: pip install -r requirements.txt pytest\n      - run: python -m pytest -W error::DeprecationWarning<\/code><\/pre>\n<div class=\"dd-note\"><b>Note:<\/b> <code>allow-prereleases: true<\/code> lets <code>setup-python<\/code> pick the release candidate until the final 3.15.0 is published, then it uses the stable version automatically.<\/div>\n<p>For the complete list of changes, read the official <a href=\"https:\/\/docs.python.org\/3.15\/whatsnew\/3.15.html\" target=\"_blank\" rel=\"noopener\">What&#8217;s New in Python 3.15<\/a> page.<\/p>\n<\/div>\n\n\n\n<div class=\"dd-post\">\n<h2 id=\"faq\">Frequently Asked Questions<\/h2>\n<details>\n<summary>When is Python 3.15 released?<\/summary>\n<p>The final Python 3.15.0 release is scheduled for October 9, 2026, according to <a href=\"https:\/\/peps.python.org\/pep-0790\/\" target=\"_blank\" rel=\"noopener\">PEP 790<\/a>. Release Candidate 3 came out on October 2, 2026. Python 3.15 will get bugfix releases for about two years and security fixes until around October 2031.<\/p>\n<\/details>\n<details>\n<summary>What are the most important Python 3.15 new features?<\/summary>\n<p>Explicit lazy imports (<code>lazy import x<\/code>), the built-in <code>frozendict<\/code>, unpacking in comprehensions (<code>[*x for x in xs]<\/code>), UTF-8 as the default encoding, the Tachyon sampling profiler in the new <code>profiling<\/code> package, the <code>sentinel<\/code> built-in, TypedDict <code>extra_items<\/code>\/<code>closed<\/code>, better error messages and a faster JIT.<\/p>\n<\/details>\n<details>\n<summary>Do lazy imports make my web app faster?<\/summary>\n<p>They mainly reduce <em>startup time<\/em> and memory for code paths that never use a module. CLIs, serverless functions and test runs gain the most. A FastAPI or Django server that runs for hours imports everything once, so request speed is unchanged \u2014 but faster restarts and cold starts still help.<\/p>\n<\/details>\n<details>\n<summary>Is Python 3.15 still using the GIL?<\/summary>\n<p>Yes, the default build still has the GIL. The free-threaded (no-GIL) build has been officially supported since Python 3.14 and is available as a separate build; the macOS installer for 3.15 now includes it by default as an option.<\/p>\n<\/details>\n<details>\n<summary>Should I upgrade my production app to Python 3.15 immediately?<\/summary>\n<p>Start testing now, but upgrade production once all your key dependencies publish 3.15 wheels and your test suite is green. Many teams wait for the first one or two bugfix releases (3.15.1\/3.15.2).<\/p>\n<\/details>\n<details>\n<summary>Will UTF-8 default break my existing code?<\/summary>\n<p>Only code that silently relied on a non-UTF-8 locale encoding, mostly on Windows. Fix it by passing <code>encoding=\"locale\"<\/code> or the exact encoding, or temporarily run with <code>-X utf8=0<\/code>.<\/p>\n<\/details>\n\n<h2 id=\"conclusion\">Conclusion<\/h2>\n<p>Python 3.15 is a practical, &#8220;quality of life&#8221; release. Lazy imports solve a real startup problem without hacks, <code>frozendict<\/code> and <code>sentinel<\/code> fill long-standing gaps in the built-ins, UTF-8 by default removes a classic cross-platform bug, and Tachyon gives every developer a production-grade profiler for free. Install the release candidate in a virtual environment, add 3.15 to your CI matrix, and try <code>-X lazy_imports=all<\/code> on your project today \u2014 you may get a faster app with zero code changes.<\/p>\n\n<div class=\"dd-cta\">\n<h3>New posts every day on DevDojo<\/h3>\n<p>Practical tutorials on Python, FastAPI, Node.js, React, Next.js and AI \u2014 written for beginner and intermediate developers.<\/p>\n<p>Bookmark <a href=\"https:\/\/devdojo.co.in\/\">devdojo.co.in<\/a>, and tell us in the comments which Python 3.15 feature you&#8217;ll use first!<\/p>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"<p>Python 3.15 new features explained with code: lazy imports, frozendict, comprehension unpacking, UTF-8 default, Tachyon profiler and upgrade tips.<\/p>\n","protected":false},"author":1,"featured_media":169,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7,9],"tags":[56,54,53,52,51,55],"class_list":["post-170","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-backend","category-python","tag-backend-development","tag-frozendict","tag-lazy-imports","tag-python","tag-python-3-15","tag-python-profiling"],"aioseo_notices":[],"aioseo_head":"\n\t\t<!-- All in One SEO 5.0.2.1 - aioseo.com -->\n\t<meta name=\"description\" content=\"Python 3.15 new features explained with code: lazy imports, frozendict, comprehension unpacking, UTF-8 default, Tachyon profiler and upgrade tips.\" \/>\n\t<meta name=\"robots\" content=\"max-image-preview:large\" \/>\n\t<meta name=\"author\" content=\"Lokendra\"\/>\n\t<link rel=\"canonical\" href=\"https:\/\/devdojo.co.in\/index.php\/2026\/10\/05\/python-3-15-new-features\/\" \/>\n\t<meta name=\"generator\" content=\"All in One SEO (AIOSEO) 5.0.2.1\" \/>\n\t\t<meta property=\"og:locale\" content=\"en_US\" \/>\n\t\t<meta property=\"og:site_name\" content=\"DevDojo - Empowering Developers\" \/>\n\t\t<meta property=\"og:type\" content=\"article\" \/>\n\t\t<meta property=\"og:title\" content=\"Python 3.15 New Features: Lazy Imports, frozendict &amp; 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