python - 为什么 pdb 为列表理解的 "if"子句中引用的变量提供 NameError？

Question

为什么会这样 NameError发生？

(为了便于阅读，插入提示前的空白行。)

$ python3
Python 3.4.10 (default, Oct  4 2019, 19:39:58)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-23)] on linux
Type "help", "copyright", "credits" or "license" for more information.

>>> import pdb

>>> def blah():
...     foo = "ab"
...     pdb.set_trace()
... 

>>> blah()
--Return--
> <stdin>(3)blah()->None

(Pdb) [bar for bar in "ac" if bar in foo]
*** NameError: name 'foo' is not defined

以下所有工作正常:

(Pdb) foo
'ab'

(Pdb) [bar for bar in foo]
['a', 'b']

(Pdb) [bar for bar in "ac" if bar in "ab"]
['a']

因此，对于 if 中引用的变量，这是一个特别的问题。子句，而不是循环变量本身。

在 python 3.6.9 (default, Apr 18 2020, 01:56:04) 中也可以看到与上述相同的行为.

但是在 python 2(2.6.6 或 2.7.17)中，使用与上面相同的命令，我得到:

(Pdb) [bar for bar in "ac" if bar in foo]
['a']

Answer 1

List comprehensions in Python3 are evaluated with a separate scope, similar to functions.只有第一个可迭代对象 ( for ... in iterable ) 是从外部注入(inject)的，所有其他名称都绑定(bind)为全局变量或闭包。 However, closures are resolved at compile time – they only work when lexically defined in the outer scope containing a name.
将此与定义“内部”函数进行比较，该函数显示相同的行为:

-> pdb.set_trace()
(Pdb) foo
'ab'
(Pdb) def bar(): return foo
(Pdb) bar()
*** NameError: name 'foo' is not defined

在 PDB session 中，执行范围在 blah 内。 ，但词法范围在外面。这类似于在其父函数内部或外部定义“内部”函数/理解:

# lexically scoped `foo` as closure
def blah():
    foo = 3
    def bar():  # !!! Define new scope inside blah
        print(foo)
    bar()  # execute bar inside blah

# `foo` not in scope
def bar():  # !!! Define new scope outside blah
    print(foo)

def blah():
    foo = 3
    bar()  # execute bar inside blah

6.2.4. Displays for lists, sets and dictionaries

[...]

However, aside from the iterable expression in the leftmost forclause, the comprehension is executed in a separate implicitly nestedscope. This ensures that names assigned to in the target list don’t“leak” into the enclosing scope.

4.2.2. Resolution of names

[...]

When a name is used in a code block, it is resolved using the nearestenclosing scope. The set of all such scopes visible to a code block iscalled the block’s environment.