`del` on a package has some kind of memory

问题

del seems to have some memory which puzzles me. See the following:

In [1]: import math

In [2]: math.cos(0)
Out[2]: 1.0

In [3]: del math.cos

In [4]: math.cos(0)
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-9cdcc157d079> in <module>()
----> 1 math.cos(0)

AttributeError: module 'math' has no attribute 'cos'

Fine. Let's see what happens if we delete the whole math package:

In [5]: del math

In [6]: math.cos(0)
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-6-9cdcc157d079> in <module>()
----> 1 math.cos(0)

NameError: name 'math' is not defined

So now math itself is gone, as expected.

Now let's import math again:

In [7]: import math

In [8]: math.cos(0)
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-8-9cdcc157d079> in <module>()
----> 1 math.cos(0)

AttributeError: module 'math' has no attribute 'cos'

So somehow interactive python remembers that math.cos was deleted specifically even after we deleted the whole math package and imported it again.

Where does python keeps this knowledge? Can we access it? Can we change it?

回答1:

I would say that the package is still seen as imported. So performing import math again just redeclares the name, but with old contents.

You could use reload to make sure your module is whole again, except that some versions of python require to remove the entry in sys.modules as well, which makes the use of reload redundant:

import math
del math.cos
del math
sys.modules.pop("math")   # remove from loaded modules
import math
print(math.cos(0))  # 1.0

(this difference between various python versions, reload and import are discussed in a follow-up question: Should importlib.reload restore a deleted attribute in Python 3.6?)

回答2:

A package is only read from disk once and then stored in memory as mutable singleton. The second time you import it you get the exact same singleton you have previously imported, and it's still missing its cos. del math merely deletes the local name for it, it doesn't "unimport" the package from Python overall.

回答3:

del math does not delete the package at all, it just deletes the local name math in the current module.

Like any other object, if any other references to the math module exist anywhere, then it's kept in memory.

And in particular, sys.modules is always a dictionary of all loaded modules, so at least there's always a reference there.

Edit: But there's a way to actually reload a module, imp.reload.

Unfortunately I can't get it to work for this case, reload needs the random module (probably to create some part of the compiled Python file), the random module needs math.cos, and it's gone. Even with importing random first there is no error, but math.cos doesn't reappear; I don't know why, maybe because it's a builtin module.

来源：https://stackoverflow.com/questions/48809458/del-on-a-package-has-some-kind-of-memory