btrees

Ensure you install the correct package and import from it: `pip install BTrees` followed by `from BTrees.OOBTree import OOBTree` (or other specific BTree types).

To customize node sizes, you should subclass the BTree type and set `max_internal_size` and `max_leaf_size` within the subclass definition. Alternatively, in `btrees` versions 4.9.0 and later, you can modify these attributes directly on the class if using the pure Python implementation or via a specific mechanism if using the C extension.
```python
# Recommended way to customize node sizes
import BTrees.OOBTree

class MyCustomBTree(BTrees.OOBTree.BTree):
    max_leaf_size = 500
    max_internal_size = 1000

my_tree = MyCustomBTree()
```
Or, if the version supports it and you intend to modify global defaults (check documentation for specific version behavior):
```python
# This might work in newer versions for C extensions or pure-Python implementations
import BTrees.OOBTree
BTrees.OOBTree.BTree.max_internal_size = 1000 
```

Ensure that any custom object used as a key in a BTree implements a reliable comparison method, such as `__lt__` (less than) to provide a total ordering. For persistent objects in ZODB, avoid using `Persistent` objects as keys directly without careful consideration of their comparison behavior.
```python
import BTrees.OOBTree

class MySortableObject:
    def __init__(self, value):
        self.value = value

    def __lt__(self, other):
        if isinstance(other, MySortableObject):
            return self.value < other.value
        return NotImplemented

    def __eq__(self, other):
        if isinstance(other, MySortableObject):
            return self.value == other.value
        return NotImplemented

    def __hash__(self):
        return hash(self.value) # Required if objects are also used in sets/dicts

bt = BTrees.OOBTree.BTree()
bt[MySortableObject(1)] = 'one'
bt[MySortableObject(2)] = 'two'
```

Avoid modifying a BTree (or any of its constituent parts) during iteration. If modifications are necessary, collect the keys to be modified/deleted beforehand and then perform the operations in a separate loop after the iteration, or create a copy of the keys/items to iterate over. If the error persists without explicit concurrent modification, it may indicate data corruption, potentially requiring diagnostic tools like `BTrees.check.check()` or database recovery procedures if used with ZODB.
```python
import BTrees.OOBTree

bt = BTrees.OOBTree.BTree()
bt['a'] = 1
bt['b'] = 2
bt['c'] = 3

# INCORRECT (will raise RuntimeError if 'd' is added while iterating)
# for key in bt.keys():
#     if key == 'b':
#         bt['d'] = 4

# CORRECT way to modify during 'iteration' logically
keys_to_process = list(bt.keys())
for key in keys_to_process:
    if key == 'b':
        bt['d'] = 4

# If corruption is suspected:
# from BTrees.check import check, display
# check(bt) # Raises AssertionError on consistency issues
# display(bt) # Prints internal structure for manual inspection
```