第551页 | Learning Python | 阅读 ‧ 电子书库

reload May Not Impact from Imports

Here’s another from-related gotcha: as discussed previously, because from copies (assigns) names when run, there’s no link back to the modules where the names came from. Names imported with from simply become references to objects, which happen to have been referenced by the same names in the importee when the from ran.

Because of this behavior, reloading the importee has no effect on clients that import its names using from. That is, the client’s names will still reference the original objects fetched with from, even if the names in the original module are later reset:

from module import X          # X may not reflect any module reloads!
 . . .
from imp import reload
reload(module)                # Changes module, but not my names
X                             # Still references old object

To make reloads more effective, use import and name qualification instead of from. Because qualifications always go back to the module, they will find the new bindings of module names after reloading:

import module                 # Get module, not names
 . . .
from imp import reload
reload(module)                # Changes module in-place
module.X                      # Get current X: reflects module reloads