What is the source of Mary Poppins' powers?

Question

In the movie she seems to sit on a cloud, exhibits magical powers, and is sent to help a family come back together. She seems to be operating on a mission of sorts as she helps a lot of people. Bert in the movie is actually multiple characters in the stories which Mary Poppins has been associated with. She also doesn't appear to age, as the grownup Bert has known her presumably since childhood (and they are roughly the same age).

I always assumed she was some heavenly being (such as an angel).

[UPDATE]
The title of the question has been changed to hopefully improve the question.

Answer 1

According to the books, all infants have magical powers but Mary Poppins is the only one who retains these powers into adulthood. Hence she is referred to as "The Great Exception" by a jackdaw.

Source: Wikipedia.

Answer 2

@Wikis seems to have the core answer, but it is important to read the books, not just rely on the film. The film stands alone as a fantastic work (it is one of my all time greatest films), but the books have a far darker side to them, which makes a difference in terms of how she it properly understood.

At heart, she seems to be able to do the right thing at the right time. She is a combination of the wish-fulfillment of the children and their parents - firm but fun. So her "powers" seem more like stimulating the children's imaginations than anything else, but that is, in itself, a significant achievement.

Answer 3

I always consider books and their adaptations as different stories. What works with one doesn't work with the other.

In that case, we have no explanation to the powers of Mary in the film, because the explanation given in the books doesn't work. This idea of Mary being an angel is comical, but I'm not sure it really works. She describe herself as "a perfect person", and she seems very human, as she seems able to be in love with Bert (yes, by the way, this idea of Bert having known Mary during his childhood doesn't convince me so much). Perhaps she's a kind of magician, a person who is basically human but who won/was born with some powers, and that, as a spiritually perfect person, decides to use it for the Good.

Answer 4

What is the source of Mary Poppins' powers?

Your belief in her.

I always assumed she was some heavenly being (such as an angel).

Following, the previous sentences, if that's what you believe her to be, then for you she is a heavenly being, like an angel.

The notion that something exists if you believe in it was very popular in children's movies and books of the time. I think I remember a Disney movie with a car traveling through a forest inhabited with gnomes with a similar idea. And tons of others: Peter Pan, Bedknobs and Broomsticks, you name it...