Exam task for Darya Nemirich

Your program should never fail.

You are free to implement the task as you want. In particular, if there are some uncertainties, you have to decide what to do.

You are free to add some functionality. Please describe all features you added.

You should store information about flights in file flights.av.

Each item has several properties:

You can assume that:

All time properties have the same timezone
There is no flight that spans across two days (Departure < Arrival is always true)
Each flight happens every day

Passengers can travel from one airport to another with transfers. But our service will provide an opportunity to make at most one transfer:

It is impossible to make a transfer to a flight such that its departure is earlier than first flight's departure plus 40 minutes

It is possible to make second flight at the next day after the first one:

  Saint-Petersburg 22:20 -> Moscow 23:50
  Moscow           02:40 -> Tokyo 10:15

Add: python avia.py add <From> <To> <Departure> <Arrival> <Cost>

Command should add new flight with the given properties.
Find: python avia.py find <From> <To>

Command should find a way from <From> airport to <To> airport for a passenger. Passenger can transfer at most once. It is always better to make no transfer, so if it is possible, find the fastest way to flight from <From> to <To> without transfer. Otherwise find the fastest way with exactly one transfer. If it is impossible too, print a message that it is impossible to travel from <From> to <To>. Your message should be pretty, for example:
```
Saint-Petersburg 12:20 -> Moscow 13:40
Moscow           14:40 -> Tokyo 22:20
Cost:  $450
```
Find cheap: python avia.py find-cheap <From> <To>

Command should find the cheapest way from <From> to <To> with at most one transfer. The format of output is the same as in previous command.

dnemirich / python_course