Loop on all errors in `InteractiveMigrationQuestioner._ask_default()`

[Adam Johnson]

When changing some fields, the migration questioner can ask for new field default values entered through a Python prompt.

Yesterday, I made a typo which made the makemigrations process exit. This was a little frustrating as there were several fields to fix, and my work answering for the first fields was lost.

Currently, the questioner loops on SyntaxError and NameError (​source). My mistake lead to an AttributeError, hence the crash and lost work. I propose we extend this clause to Exception - there's little harm in displaying the error and looping.

Here's a reproducing example. Say you drop null=True from two model fields:

example/models.py

@@ -2 +2 @@
 
 
 class Book(models.Model):
-    title = models.CharField(max_length=100, null=True)
-    published_date = models.DateField(null=True)
+    title = models.CharField(max_length=100)
+    published_date = models.DateField()

makemigrations asks you about the first one:

$ ./manage.py makemigrations example
It is impossible to change a nullable field 'title' on book to non-nullable without providing a default. This is because the database needs something to populate existing rows.
Please select a fix:
 1) Provide a one-off default now (will be set on all existing rows with a null value for this column)
 2) Ignore for now. Existing rows that contain NULL values will have to be handled manually, for example with a RunPython or RunSQL operation.
 3) Quit and manually define a default value in models.py.
Select an option: 1
Please enter the default value as valid Python.
The datetime and django.utils.timezone modules are available, so it is possible to provide e.g. timezone.now as a value.
Type 'exit' to exit this prompt
>>> "Unknown book"

Immediately after this, it asks about the second field. Making a typo like .dat() instead of .date() crashes the process:

It is impossible to change a nullable field 'published_date' on book to non-nullable without providing a default. This is because the database needs something to populate existing rows.
Please select a fix:
 1) Provide a one-off default now (will be set on all existing rows with a null value for this column)
 2) Ignore for now. Existing rows that contain NULL values will have to be handled manually, for example with a RunPython or RunSQL operation.
 3) Quit and manually define a default value in models.py.
Select an option: 1
Please enter the default value as valid Python.
The datetime and django.utils.timezone modules are available, so it is possible to provide e.g. timezone.now as a value.
Type 'exit' to exit this prompt
>>> datetime.dat(1970, 1, 1)
Traceback (most recent call last):
  File "/..././manage.py", line 21, in <module>
    main()
  ...
  File "/.../.venv/.../django/db/migrations/questioner.py", line 162, in _ask_default
    return eval(code, {}, {"datetime": datetime, "timezone": timezone})
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<string>", line 1, in <module>
AttributeError: module 'datetime' has no attribute 'dat'. Did you mean: 'date'?

But if you instead made a typo in the datetime name, it would loop:

>>> datetme.date(1970,1,1)
Invalid input: name 'datetme' is not defined
>>>

[Simon Charette]

As long as KeyboardInterrupt(BaseException) is allowed to bubble up (which catching for Exception allows) I agree that not trying to catch a predefined set of exceptions is a better default.

[Sarah Boyce <42296566+sarahboyce@…>]

In 3434fab7:

Refs #35882 -- Added test for migration questioner KeyboardInterrupt.

[Sarah Boyce <42296566+sarahboyce@…>]

In e035db1:

Fixed #35882 -- Made migration questioner loop on all errors.