問題描述

根據python參考手冊我們有

當根本找不到名稱時，會引發 NameError 異常.如果該名稱指的是尚未綁定的局部變量，a引發 UnboundLocalError 異常.UnboundLocalError 是一個子類的名稱錯誤.

When a name is not found at all, a NameError exception is raised. If the name refers to a local variable that has not been bound, a UnboundLocalError exception is raised. UnboundLocalError is a subclass of NameError.

我不明白什么時候拋出 UnboundLocalError?因為

I don't to understand when the UnboundLocalError is thrown? Because

Python 缺少聲明并允許發生名稱綁定操作代碼塊內的任何位置.

Python lacks declarations and allows name binding operations to occur anywhere within a code block.

那么我們怎樣才能聲明一個變量，而不是初始化她呢?

So how we can declare a variable, but not to initialize her?

推薦答案

你可以引用一個沒有分配的名字:

You can refer to a name without having assigned to it:

>>> foobar
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'foobar' is not defined

這里 foobar 被引用，但從未被分配到.這會引發 NameError 因為名稱從未綁定.

Here foobar is being referred to, but was never assigned to. This raises a NameError because the name was never bound.

更微妙的是，這里沒有發生分配，因為執行的行永遠不會運行:

More subtly, here assignment is not happening because the line that does is never run:

>>> def foo():
...     if False:
...         spam = 'eggs'
...     print spam
... 
>>> foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in foo
UnboundLocalError: local variable 'spam' referenced before assignment

因為 spam = 'eggs' 永遠不會執行，所以 print spam 會引發 UnboudLocalError.

Because spam = 'eggs' is never executed, print spam raises an UnboudLocalError.

請注意，Python 中沒有任何地方聲明過.您綁定或不綁定，聲明不是語言的一部分.

Note that nowhere in Python is a name ever declared. You bind or don't bind, declaration is not part of the language.

相反，binding 用于確定名稱的范圍；綁定操作包括賦值、用于 for 循環的名稱、函數參數、導入語句、在 except 子句中保存捕獲的異常的名稱、上下文管理器的名稱一個 with 語句所有的綁定名稱.

Instead, binding is used to determine the scope of a name; binding operations include assignment, names used for a for loop, function parameters, import statements, name to hold a caught exception in an except clause, the name for a context manager in a with statement all bind names.

如果名稱綁定在范圍內(例如在函數中)，則它是本地名稱，除非您使用 global 語句(或 nonlocal 語句在 Python 3 中)將名稱顯式標記為全局(或閉包).

If a name is bound in a scope (such as in a function) then it is a local name, unless you use a global statement (or a nonlocal statement in Python 3) to explicitly mark the name as a global (or a closure) instead.

所以下面是一個錯誤:

>>> foo = None
>>> def bar():
...     if False:
...         foo = 'spam'
...     print foo
... 
>>> bar()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in bar
UnboundLocalError: local variable 'foo' referenced before assignment

因為 foo 被 somewhere 綁定在 bar 函數范圍內.但是如果您將 foo 標記為全局，則該函數可以工作:

because foo is being bound somewhere in the bar function scope. But if you mark foo as a global, the function works:

>>> foo = None
>>> def bar():
...     global foo
...     if False:
...         foo = 'spam'
...     print foo
... 
>>> bar()
None

因為現在 Python 編譯器知道您希望 foo 改為全局變量.

because now the Python compiler knows you wanted foo to be a global instead.

所有這些都記錄在 命名和綁定部分 Python 參考文檔.

This is all documented in the Naming and Binding section of the Python reference documentation.

這篇關于未綁定的變量和名稱的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網！