Redefinition - Dependency Injection Alternative?

Ultimately, what is dependency injection? The option to redefine the semantics of some object.

Take the following traditional approach:

def foo():
    print('x')

def bar(foo_impl=None):
    if not foo_impl:
        foo_impl = foo

    foo_impl()

def baz():
    foo = lambda: print('y')
    bar(foo_impl=foo)

baz()   # prints 'y'

Generally there is some sort of singleton (e.g. a database reference) that the code will default to in production. Since this singleton is in scope, Python’s option to redefine global symbols allows redefinition without having to touch any dependent functions:

def foo():
    print('x')

def bar():
    foo()

def baz():
    global foo
    foo = lambda: print('y')
    bar()

baz()   # prints 'y'

The pitfall: all functions dependent on foo will now use this new definition. This seems acceptable as long as tests are isolated. Presumably there are DI frameworks that can perform runtime modifications that can accomplish this in other languages as well..?