Find and purge unused blocks

[mozman]

An often asked feature is to find and remove unused (unreferenced) block definitions.

The ezdxf.blkrefs module provides a block reference counter for this task:

import ezdxf
from ezdxf import blkrefs

doc = ezdxf.readfile("your.dxf")
ref_counter = blkrefs.BlockReferenceCounter(doc)

unreferenced_blocks = set()
for block in doc.blocks:
    if not block.is_alive or block.is_any_layout:
        continue
    count = ref_counter.by_name(block.name)
    if count == 0:
        # DO NOT DELETE BLOCK WHILE ITERATING!
        unreferenced_blocks.add(block.name)

for block_name in unreferenced_blocks:
    # The blkrefs.BlockReferenceCounter counts ALL KNOWN references, it should be safe 
    # to destroy these blocks without safeguards (no guarantee for that!):
    doc.blocks.delete_block(block_name, safe=False)

Setting safe to False is important to remove unused anonymous blocks such as those used by the DIMENSION entity. The delete_block() method does not delete anonymous blocks in safe mode because finding all references is costly and is not implemented.

I added the new function ezdxf.blkrefs.find_unreferenced_blocks() in v1.3.5 to simplify this task:

import ezdxf
from ezdxf import blkrefs

doc = ezdxf.readfile("your.dxf")

for block_name in blkrefs.find_unreferenced_blocks(doc):
    doc.blocks.delete_block(block_name, safe=False)