A higher-level topological sort.

## Project description

whatwhen is a higher-level topological sort, a useful algorithm to resolve dependencies and calculate a workable execution order for functions and other things.

Basic topological sorting:

```tasks = [
{
'name: 'drive',
'dependencies': ['car'],
}, {
'name': 'car',
'dependencies': ['wheels', 'gas'],
}, {
'name': 'wheels',
'dependencies': 'rubber',
}, {
'name': 'gas',
}, {
'name': 'rubber',
},
]

whatwhen.sort(items)
```

Topological sorting using contracts (needs and provisions):

```tasks = [
{
'name': 'drive',
'needs': ['car', 'gas'],
}, {
'name': 'fill tank',
'provides': ['gas'],
}, {
'name': 'rent a car',
'provides': ['car'],
'dependencies': ['money'],
}, {
'name': 'money'
}
]

whatwhen.sort(items)
```

Needs and provisions are useful when you don’t know exactly what function (or system or module or whatever) is going to provide the necessary data or preconditions, but you do know what that data or those preconditions are. This might happen in large systems or systems over which you don’t have full control.

```def one():
pass

def two():
pass

one.dependencies = ['two']

whatwhen.sort([one, two])
```

Whatwhen will return the original functions (or dictionaries) in sorted order, rather than just a sorted list of node names, so it’s slightly easier to work with than a raw topological sort.

Whatwhen lets the tarjan module do most of the heavy lifting, so there’s really only about 40 lines of wrapper code to this module. It’s up to you to decide whether you’d rather build on top of WhatWhen or plain Tarjan.