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The new pythonista_startup feature is nice. The fact that any globals coming from pythonista_startup aren't removed is also nice. Especially because I can now do this:

from __future__ import division, print_function

import sys
import types

class ContainAllTheThings(list):
    def __getitem__(self, key):
        try:
            return super(ContainAllTheThings, self).__getitem__(key)
        except (IndexError, KeyError):
            return None

    def __contains__(self, obj):
        return super(ContainAllTheThings, self).__contains__(obj) or True

class DirAllTheThings(types.ModuleType):
    def __dir__(self):
        return ContainAllTheThings()

new_module = DirAllTheThings(__name__, __doc__)
vars(new_module).update(vars(sys.modules["pythonista_startup"]))
sys.modules["pythonista_startup"] = new_module

(Can't we just have an option to keep all globals? ;))

Hehe, very clever! :)

Can you explain it? can't tell what it does.

@Webmaster4o said:

Can you explain it? can't tell what it does.

Every time you run a script, Pythonista deletes all global variables, except for those that come from the pythonista_startup module (which is run automatically when the app is started). Internally this is done by looking at dir(pythonista_startup) - any names listed there are not deleted from the globals. I'm abusing this feature just a tiny bit so that no globals are ever removed.

The first class, ContainAllTheThings, is a subclass of list that pretends to contain every object, i. e. obj in ContainAllTheThings() is always True for any object. The second class, DirAllTheThings, is a subclass of module (aka types.ModuleType) with a modified __dir__ method that returns a ContainAllTheThings object. This means that obj in dir(DirAllTheThings()) is always True. Finally we replace the real module pythonista_startup with a DirAllTheThings object and copy all attributes over.

To find out which global names come from pythonista_startup, Pythonista internally checks name in dir(pythonista_startup) for every name. Now we've replaced pythonista_startup with an instance of DirAllTheThings, meaning that when we call dir on it we get an instance of ContainAllTheThings. Now when Pythonista checks whether a name comes from pythonista_startup, the answer is always True, meaning that no name is ever deleted.

Ah. Clever!

The original version no longer works in Pythonista 3. It seems that the dir function now converts the return value of __dir__ to an actual list, instead of allowing a subclass. This is the updated version:

import sys
import types

class DirAllTheGlobals(types.ModuleType):
    import __main__

    def __dir__(self):
        return dir(type(self).__main__)

# THESE LINES MUST COME LAST.
# Anything past this point is executed in the context of the old
# pythonista_startup module, which may already be partially
# garbage-collected.
new_module = DirAllTheGlobals(__name__, __doc__)
vars(new_module).update(vars(sys.modules["pythonista_startup"]))
sys.modules["pythonista_startup"] = new_module

This should also still work in Pythonista 2.

@omz please can you add the option to keep globals back?