# Python Shelve Module

The `shelve` module implements a persistent, dictionary-like object. It is backed by a database file (using `dbm`), allowing you to store Python objects that can be pickled.

## Importing the Module

```python
import shelve
```

## Opening a Shelf

To open a shelf, use `shelve.open()`. It takes a filename as an argument.

```python
import shelve

# Open a shelf file
with shelve.open('mydata') as shelf:
    # ... use the shelf ...
    pass
```

## Writing Data

You can treat the shelf object like a dictionary. Keys must be strings, but values can be any picklable Python object.

```python
import shelve

with shelve.open('mydata') as shelf:
    shelf['key1'] = {1, 2, 3}
    shelf['key2'] = 'some string'
    shelf['key3'] = [10, 20, 30]
```

## Reading Data

Reading is also like accessing a dictionary.

```python
import shelve

with shelve.open('mydata') as shelf:
    print(shelf['key1'])
    # Output: {1, 2, 3}
    
    if 'key2' in shelf:
        print(shelf['key2'])
```

## Updating Data

By default, `shelve` does not track modifications to mutable objects stored in the shelf. To ensure changes are saved, you can either re-assign the object or set `writeback=True`.

### Without `writeback=True`

```python
with shelve.open('mydata') as shelf:
    data = shelf['key3']
    data.append(40)
    shelf['key3'] = data # Explicitly write back
```

### With `writeback=True`

```python
with shelve.open('mydata', writeback=True) as shelf:
    shelf['key3'].append(50) # Automatically saved
```

*Note: `writeback=True` can be slower and consume more memory because it caches all accessed entries.*

[[programming/python/python]]