# Python Graphlib Module

The `graphlib` module, introduced in Python 3.9, provides functionality to perform topological sorting of a graph of hashable nodes.

## Importing the Module

```python
import graphlib
```

## Topological Sorting

A topological sort of a directed graph is a linear ordering of its vertices such that for every directed edge `u -> v` from vertex `u` to vertex `v`, `u` comes before `v` in the ordering.

### `TopologicalSorter`

The `TopologicalSorter` class allows you to perform topological sorting. You can initialize it with a dictionary where keys are nodes and values are iterables of their predecessors (dependencies).

```python
import graphlib

# Create a graph: key depends on values (predecessors)
graph = {
    "D": {"B", "C"},
    "C": {"A"},
    "B": {"A"}
}

ts = graphlib.TopologicalSorter(graph)
```

### `static_order()`

Returns an iterable of nodes in a topological order.

```python
print(list(ts.static_order()))
# Output: ['A', 'C', 'B', 'D'] (Order of C and B may vary)
```

### Step-by-Step Execution

For more control (e.g., parallel processing), you can use `prepare()`, `get_ready()`, and `done()`.

```python
ts = graphlib.TopologicalSorter(graph)
ts.prepare()

while ts.is_active():
    ready_nodes = ts.get_ready()
    print(f"Ready: {ready_nodes}")
    
    # Process nodes...
    for node in ready_nodes:
        ts.done(node)
```

### Handling Cycles

If the graph contains a cycle, `static_order()` raises a `CycleError`.

[[programming/python/python]]