在 Python 中，为什么 pow 这样的函数可以直接调用，而 floor 这样的函数得先导入模块？

python新手，望大神们多指教

回复内容：

本来写在 @bhuztez 大大的回答的评论里的。算了还是发到顶层好了。

诶这里 @蓝色 大大的回答真的误解一些了， @bhuztez 大大的回答才是完全正解。

这个问题，要看最精辟的答案的请看 @flow memory 大大的，要深入到Python的内部机制的请看 @bhuztez 大大的，要看点具体代码的请看 @蓝色 大大修正过的答案。

Python的__builtin___模块完全是个运行时的东西， @蓝色 大大引用的代码其实是在VM初始化的时候把初始的__builtin___模块中的名字与函数指针的对应关系注册好；然而Python的（源码到字节码）编译器是不关心这个的。
Python的pow()跟像GCC的__builtin_powi()不可以相提并论；前者的行为可以在运行时改变，而编译器完全不把它当作特殊的东西看待；后者则是编译器直接支持的intrinsic function。

刚初始化好的时候，__builtin__模块里的"pow"映射到的是builtin_pow()函数，后者进一步调用PyNumber_Power()函数来实现功能；cpython/bltinmodule.c at 2.7 · python/cpython · GitHub
Python的源码编译器会把 ** 运算符编译为BINARY_POWER字节码指令，而Python的字节码解释器为BINARY_POWER的实现则是直接调用PyNumber_Power()函数（不通过符号解析__builtin__模块里的"pow"的当前绑定）。cpython/ceval.c at 2.7 · python/cpython · GitHub

由于在Python代码里调用pow()实际上要先经过一次符号解析（LOAD_NAME）找到目标然后再调用过去，而模块的绑定又是可变的，所以可以做到下面这种事情：

<span class="err">$</span> <span class="n">python</span>
<span class="n">Python</span> <span class="mf">2.7</span><span class="o">.</span><span class="mi">5</span> <span class="p">(</span><span class="n">default</span><span class="p">,</span> <span class="n">Mar</span>  <span class="mi">9</span> <span class="mi">2014</span><span class="p">,</span> <span class="mi">22</span><span class="p">:</span><span class="mi">15</span><span class="p">:</span><span class="mo">05</span><span class="p">)</span> 
<span class="p">[</span><span class="n">GCC</span> <span class="mf">4.2</span><span class="o">.</span><span class="mi">1</span> <span class="n">Compatible</span> <span class="n">Apple</span> <span class="n">LLVM</span> <span class="mf">5.0</span> <span class="p">(</span><span class="n">clang</span><span class="o">-</span><span class="mf">500.0</span><span class="o">.</span><span class="mi">68</span><span class="p">)]</span> <span class="n">on</span> <span class="n">darwin</span>
<span class="n">Type</span> <span class="s">"help"</span><span class="p">,</span> <span class="s">"copyright"</span><span class="p">,</span> <span class="s">"credits"</span> <span class="ow">or</span> <span class="s">"license"</span> <span class="k">for</span> <span class="n">more</span> <span class="n">information</span><span class="o">.</span>
<span class="o">>>></span> <span class="mi">2</span> <span class="o">**</span> <span class="mi">5</span>
<span class="mi">32</span>
<span class="o">>>></span> <span class="nb">pow</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">5</span><span class="p">)</span>
<span class="mi">32</span>
<span class="o">>>></span> <span class="n">__builtins__</span>
<span class="o"><</span><span class="n">module</span> <span class="s">'__builtin__'</span> <span class="p">(</span><span class="n">built</span><span class="o">-</span><span class="ow">in</span><span class="p">)</span><span class="o">></span>
<span class="o">>>></span> <span class="nb">dir</span><span class="p">(</span><span class="n">__builtins__</span><span class="p">)</span>
<span class="p">[</span><span class="s">'ArithmeticError'</span><span class="p">,</span> <span class="s">'AssertionError'</span><span class="p">,</span> <span class="s">'AttributeError'</span><span class="p">,</span> <span class="s">'BaseException'</span><span class="p">,</span> <span class="s">'BufferError'</span><span class="p">,</span> <span class="s">'BytesWarning'</span><span class="p">,</span> <span class="s">'DeprecationWarning'</span><span class="p">,</span> <span class="s">'EOFError'</span><span class="p">,</span> <span class="s">'Ellipsis'</span><span class="p">,</span> <span class="s">'EnvironmentError'</span><span class="p">,</span> <span class="s">'Exception'</span><span class="p">,</span> <span class="s">'False'</span><span class="p">,</span> <span class="s">'FloatingPointError'</span><span class="p">,</span> <span class="s">'FutureWarning'</span><span class="p">,</span> <span class="s">'GeneratorExit'</span><span class="p">,</span> <span class="s">'IOError'</span><span class="p">,</span> <span class="s">'ImportError'</span><span class="p">,</span> <span class="s">'ImportWarning'</span><span class="p">,</span> <span class="s">'IndentationError'</span><span class="p">,</span> <span class="s">'IndexError'</span><span class="p">,</span> <span class="s">'KeyError'</span><span class="p">,</span> <span class="s">'KeyboardInterrupt'</span><span class="p">,</span> <span class="s">'LookupError'</span><span class="p">,</span> <span class="s">'MemoryError'</span><span class="p">,</span> <span class="s">'NameError'</span><span class="p">,</span> <span class="s">'None'</span><span class="p">,</span> <span class="s">'NotImplemented'</span><span class="p">,</span> <span class="s">'NotImplementedError'</span><span class="p">,</span> <span class="s">'OSError'</span><span class="p">,</span> <span class="s">'OverflowError'</span><span class="p">,</span> <span class="s">'PendingDeprecationWarning'</span><span class="p">,</span> <span class="s">'ReferenceError'</span><span class="p">,</span> <span class="s">'RuntimeError'</span><span class="p">,</span> <span class="s">'RuntimeWarning'</span><span class="p">,</span> <span class="s">'StandardError'</span><span class="p">,</span> <span class="s">'StopIteration'</span><span class="p">,</span> <span class="s">'SyntaxError'</span><span class="p">,</span> <span class="s">'SyntaxWarning'</span><span class="p">,</span> <span class="s">'SystemError'</span><span class="p">,</span> <span class="s">'SystemExit'</span><span class="p">,</span> <span class="s">'TabError'</span><span class="p">,</span> <span class="s">'True'</span><span class="p">,</span> <span class="s">'TypeError'</span><span class="p">,</span> <span class="s">'UnboundLocalError'</span><span class="p">,</span> <span class="s">'UnicodeDecodeError'</span><span class="p">,</span> <span class="s">'UnicodeEncodeError'</span><span class="p">,</span> <span class="s">'UnicodeError'</span><span class="p">,</span> <span class="s">'UnicodeTranslateError'</span><span class="p">,</span> <span class="s">'UnicodeWarning'</span><span class="p">,</span> <span class="s">'UserWarning'</span><span class="p">,</span> <span class="s">'ValueError'</span><span class="p">,</span> <span class="s">'Warning'</span><span class="p">,</span> <span class="s">'ZeroDivisionError'</span><span class="p">,</span> <span class="s">'_'</span><span class="p">,</span> <span class="s">'__debug__'</span><span class="p">,</span> <span class="s">'__doc__'</span><span class="p">,</span> <span class="s">'__import__'</span><span class="p">,</span> <span class="s">'__name__'</span><span class="p">,</span> <span class="s">'__package__'</span><span class="p">,</span> <span class="s">'abs'</span><span class="p">,</span> <span class="s">'all'</span><span class="p">,</span> <span class="s">'any'</span><span class="p">,</span> <span class="s">'apply'</span><span class="p">,</span> <span class="s">'basestring'</span><span class="p">,</span> <span class="s">'bin'</span><span class="p">,</span> <span class="s">'bool'</span><span class="p">,</span> <span class="s">'buffer'</span><span class="p">,</span> <span class="s">'bytearray'</span><span class="p">,</span> <span class="s">'bytes'</span><span class="p">,</span> <span class="s">'callable'</span><span class="p">,</span> <span class="s">'chr'</span><span class="p">,</span> <span class="s">'classmethod'</span><span class="p">,</span> <span class="s">'cmp'</span><span class="p">,</span> <span class="s">'coerce'</span><span class="p">,</span> <span class="s">'compile'</span><span class="p">,</span> <span class="s">'complex'</span><span class="p">,</span> <span class="s">'copyright'</span><span class="p">,</span> <span class="s">'credits'</span><span class="p">,</span> <span class="s">'delattr'</span><span class="p">,</span> <span class="s">'dict'</span><span class="p">,</span> <span class="s">'dir'</span><span class="p">,</span> <span class="s">'divmod'</span><span class="p">,</span> <span class="s">'enumerate'</span><span class="p">,</span> <span class="s">'eval'</span><span class="p">,</span> <span class="s">'execfile'</span><span class="p">,</span> <span class="s">'exit'</span><span class="p">,</span> <span class="s">'file'</span><span class="p">,</span> <span class="s">'filter'</span><span class="p">,</span> <span class="s">'float'</span><span class="p">,</span> <span class="s">'format'</span><span class="p">,</span> <span class="s">'frozenset'</span><span class="p">,</span> <span class="s">'getattr'</span><span class="p">,</span> <span class="s">'globals'</span><span class="p">,</span> <span class="s">'hasattr'</span><span class="p">,</span> <span class="s">'hash'</span><span class="p">,</span> <span class="s">'help'</span><span class="p">,</span> <span class="s">'hex'</span><span class="p">,</span> <span class="s">'id'</span><span class="p">,</span> <span class="s">'input'</span><span class="p">,</span> <span class="s">'int'</span><span class="p">,</span> <span class="s">'intern'</span><span class="p">,</span> <span class="s">'isinstance'</span><span class="p">,</span> <span class="s">'issubclass'</span><span class="p">,</span> <span class="s">'iter'</span><span class="p">,</span> <span class="s">'len'</span><span class="p">,</span> <span class="s">'license'</span><span class="p">,</span> <span class="s">'list'</span><span class="p">,</span> <span class="s">'locals'</span><span class="p">,</span> <span class="s">'long'</span><span class="p">,</span> <span class="s">'map'</span><span class="p">,</span> <span class="s">'max'</span><span class="p">,</span> <span class="s">'memoryview'</span><span class="p">,</span> <span class="s">'min'</span><span class="p">,</span> <span class="s">'next'</span><span class="p">,</span> <span class="s">'object'</span><span class="p">,</span> <span class="s">'oct'</span><span class="p">,</span> <span class="s">'open'</span><span class="p">,</span> <span class="s">'ord'</span><span class="p">,</span> <span class="s">'pow'</span><span class="p">,</span> <span class="s">'print'</span><span class="p">,</span> <span class="s">'property'</span><span class="p">,</span> <span class="s">'quit'</span><span class="p">,</span> <span class="s">'range'</span><span class="p">,</span> <span class="s">'raw_input'</span><span class="p">,</span> <span class="s">'reduce'</span><span class="p">,</span> <span class="s">'reload'</span><span class="p">,</span> <span class="s">'repr'</span><span class="p">,</span> <span class="s">'reversed'</span><span class="p">,</span> <span class="s">'round'</span><span class="p">,</span> <span class="s">'set'</span><span class="p">,</span> <span class="s">'setattr'</span><span class="p">,</span> <span class="s">'slice'</span><span class="p">,</span> <span class="s">'sorted'</span><span class="p">,</span> <span class="s">'staticmethod'</span><span class="p">,</span> <span class="s">'str'</span><span class="p">,</span> <span class="s">'sum'</span><span class="p">,</span> <span class="s">'super'</span><span class="p">,</span> <span class="s">'tuple'</span><span class="p">,</span> <span class="s">'type'</span><span class="p">,</span> <span class="s">'unichr'</span><span class="p">,</span> <span class="s">'unicode'</span><span class="p">,</span> <span class="s">'vars'</span><span class="p">,</span> <span class="s">'xrange'</span><span class="p">,</span> <span class="s">'zip'</span><span class="p">]</span>
<span class="o">>>></span> <span class="nb">pow</span>
<span class="o"><</span><span class="n">built</span><span class="o">-</span><span class="ow">in</span> <span class="n">function</span> <span class="nb">pow</span><span class="o">></span>
<span class="o">>>></span> <span class="n">mypow</span> <span class="o">=</span> <span class="nb">pow</span>
<span class="o">>>></span> <span class="n">mypow</span>
<span class="o"><</span><span class="n">built</span><span class="o">-</span><span class="ow">in</span> <span class="n">function</span> <span class="nb">pow</span><span class="o">></span>
<span class="o">>>></span> <span class="n">__builtins__</span><span class="o">.</span><span class="n">pow</span> <span class="o">=</span> <span class="k">lambda</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">:</span> <span class="n">mypow</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span> <span class="o">-</span> <span class="mi">1</span>
<span class="o">>>></span> <span class="nb">pow</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">5</span><span class="p">)</span>
<span class="mi">31</span>
<span class="o">>>></span> <span class="mi">2</span> <span class="o">**</span> <span class="mi">5</span>
<span class="mi">32</span>

看到蓝色来冒充Python专家了，特来批判一番。这里首先是黑魔法，不然有些问题就解释不清楚啦。

>>> __builtins__
<module '__builtin__' (built-in)>
>>> pow(2,2)
4
>>> __builtins__ = None
>>> pow(2,2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'pow' is not defined
>>> __builtins__ = {'pow':1}
>>> pow
1
>>>

很多人扯一大堆道理，然而对于本问题并没什么卵用。

真正原因在于，这是Python设计者的喜好罢了，因为设计者完全可以把floor也做成可以直接调用的嘛。 pow()是built-in function,所以不需要导入.
floor()是math module的function，需要import math（from math import floor）后才能用.
不过要注意的是math module中也有个pow()，和built-in的pow()有些不同.
可以看看官方文档：
2. Built-in Functions
以及
9.2. math — Mathematical functions -------------------------------Update------------------
可以看 @RednaxelaFX 的解释，我的理解有所偏差，还是放在前面吧，让更多人看到









上面说的很清楚，pow是Builtin函数，而Builtin函数是编译器直接支持的，可以参考这个链接了解Built in函数与普通函数的不同：Intrinsic function

下面我将顺便展开说说Python中是如何实现Builtin Pow的。首先在Python中，Built in函数定义在了Bltinmodules.c这个文件中，具体的代码在：

<span class="k">static</span> <span class="n">PyMethodDef</span> <span class="n">builtin_methods</span><span class="p">[]</span>

pow 貌似比 floor 更常用 更适合作为 build-in 存在 这个问题的实质就是：为什么有的函数在__builtins__,而有的不在？ 除了scope原因之外，造成這樣scope的原因，還有一個可能是Pow 是int, floor可能是float。雖然python裡面並沒有什麼type。

このウェブサイトの声明

この記事の内容はネチズンが自主的に寄稿したものであり、著作権は原著者に帰属します。このサイトは、それに相当する法的責任を負いません。盗作または侵害の疑いのあるコンテンツを見つけた場合は、admin@php.cn までご連絡ください。

ホットAIツール

Undresser.AI Undress

リアルなヌード写真を作成する AI 搭載アプリ

AI Clothes Remover

写真から衣服を削除するオンライン AI ツール。

Undress AI Tool

脱衣画像を無料で

Clothoff.io

AI衣類リムーバー

Video Face Swap

完全無料の AI 顔交換ツールを使用して、あらゆるビデオの顔を簡単に交換できます。

もっと見る

人気の記事

アサシンのクリードシャドウズ：シーシェルリドルソリューション

3週間前 By DDD

Windows11 KB5054979の新しいものと更新の問題を修正する方法

2週間前 By DDD

アサシンクリードシャドウ - 鍛冶屋を見つけて武器と鎧のカスタマイズを解除する方法

1 か月前 By DDD

Atomfallのクレーンコントロールキーカードを見つける場所

3週間前 By DDD

<🎜>：Dead Rails-すべての課題を完了する方法

3週間前 By DDD

もっと見る

ホットツール

メモ帳++7.3.1

使いやすく無料のコードエディター

SublimeText3 中国語版

中国語版、とても使いやすい

ゼンドスタジオ 13.0.1

強力な PHP 統合開発環境

ドリームウィーバー CS6

ビジュアル Web 開発ツール

SublimeText3 Mac版

神レベルのコード編集ソフト（SublimeText3）

もっと見る

ホットトピック

Gmailメールのログイン入り口はどこですか？

7644

15

CakePHP チュートリアル

1392

52

Steamのアカウント名の形式は何ですか

91

11

Win11 Activation Key Permanent

73

19

NYTの接続はヒントと回答です

34

151

もっと見る