Tutorial for Blocks

What are Blocks?

Blocks are collections of DXF entities which can be placed multiple times as block references in different layouts and other block definitions. The block reference (Insert) can be rotated, scaled, placed in 3D space by OCS and arranged in a grid like manner, each Insert entity can have individual attributes (Attrib) attached.

Create a Block

Blocks are managed as BlockLayout objects by the BlocksSection object, every drawing has only one blocks section stored in the attribute: Drawing.blocks.

import ezdxf
import random  # needed for random placing points


def get_random_point():
    """Returns random x, y coordinates."""
    x = random.randint(-100, 100)
    y = random.randint(-100, 100)
    return x, y


# Create a new drawing in the DXF format of AutoCAD 2010
doc = ezdxf.new('R2010')

# Create a block with the name 'FLAG'
flag = doc.blocks.new(name='FLAG')

# Add DXF entities to the block 'FLAG'.
# The default base point (= insertion point) of the block is (0, 0).
flag.add_lwpolyline([(0, 0), (0, 5), (4, 3), (0, 3)])  # the flag symbol as 2D polyline
flag.add_circle((0, 0), .4, dxfattribs={'color': 2})  # mark the base point with a circle

Block References (Insert)

A block reference is a DXF Insert entity and can be placed in any layout: Modelspace, any Paperspace or BlockLayout (which enables nested block references). Every block reference can be placed, scaled and rotated individually. Scaling by negative values is mirroring.

Lets insert some random flags into the modelspace:

# Get the modelspace of the drawing.
msp = doc.modelspace()

# Get 50 random placing points.
placing_points = [get_random_point() for _ in range(50)]

for point in placing_points:
    # Every flag has a different scaling and a rotation of -15 deg.
    random_scale = 0.5 + random.random() * 2.0
    # Add a block reference to the block named 'FLAG' at the coordinates 'point'.
    msp.add_blockref('FLAG', point, dxfattribs={
        'xscale': random_scale,
        'yscale': random_scale,
        'rotation': -15
    })

# Save the drawing.
doc.saveas("blockref_tutorial.dxf")

Query all block references of block FLAG:

for flag_ref in msp.query('INSERT[name=="FLAG"]'):
    print(str(flag_ref))

When inserting a block reference into the modelspace or another block layout with different units, the scaling factor between these units should be applied as scaling attributes (xscale, …) e.g. modelspace in meters and block in centimeters, xscale has to be 0.01.

What are Attributes?

An attribute (Attrib) is a text annotation attached to a block reference with an associated tag. Attributes are often used to add information to blocks which can be evaluated and exported by CAD programs. An attribute can be visible or hidden. The simple way to use attributes is just to add an attribute to a block reference by Insert.add_attrib(), but the attribute is geometrically not related to the block reference, so you have to calculate the insertion point, rotation and scaling of the attribute by yourself.

Using Attribute Definitions

The second way to use attributes in block references is a two step process, first step is to create an attribute definition (template) in the block definition, the second step is adding the block reference by Layout.add_blockref() and attach and fill attribute automatically by the add_auto_attribs() method to the block reference. The advantage of this method is that all attributes are placed relative to the block base point with the same rotation and scaling as the block, but has the disadvantage that non uniform scaling is not handled very well. The method Layout.add_auto_blockref() handles non uniform scaling better by wrapping the block reference and its attributes into an anonymous block and let the CAD application do the transformation work which will create correct graphical representations at least by AutoCAD and BricsCAD. This method has the disadvantage of a more complex evaluation of attached attributes

Using attribute definitions (Attdef):

# Define some attributes for the block 'FLAG', placed relative
# to the base point, (0, 0) in this case.
flag.add_attdef('NAME', (0.5, -0.5), dxfattribs={'height': 0.5, 'color': 3})
flag.add_attdef('XPOS', (0.5, -1.0), dxfattribs={'height': 0.25, 'color': 4})
flag.add_attdef('YPOS', (0.5, -1.5), dxfattribs={'height': 0.25, 'color': 4})

# Get another 50 random placing points.
placing_points = [get_random_point() for _ in range(50)]

for number, point in enumerate(placing_points):
    # values is a dict with the attribute tag as item-key and
    # the attribute text content as item-value.
    values = {
        'NAME': "P(%d)" % (number + 1),
        'XPOS': "x = %.3f" % point[0],
        'YPOS': "y = %.3f" % point[1]
    }

    # Every flag has a different scaling and a rotation of +15 deg.
    random_scale = 0.5 + random.random() * 2.0
    blockref = msp.add_blockref('FLAG', point, dxfattribs={
        'rotation': 15
    }).set_scale(random_scale)
    blockref.add_auto_attribs(values)

# Save the drawing.
doc.saveas("auto_blockref_tutorial.dxf")

Get/Set Attributes of Existing Block References

See the howto: Get/Set Block Reference Attributes

Evaluate Wrapped Block References

As mentioned above evaluation of block references wrapped into anonymous blocks is complex:

# Collect all anonymous block references starting with '*U'
anonymous_block_refs = modelspace.query('INSERT[name ? "^\*U.+"]')

# Collect real references to 'FLAG'
flag_refs = []
for block_ref in anonymous_block_refs:
    # Get the block layout of the anonymous block
    block = doc.blocks.get(block_ref.dxf.name)
    # Find all block references to 'FLAG' in the anonymous block
    flag_refs.extend(block.query('INSERT[name=="FLAG"]'))

# Evaluation example: collect all flag names.
flag_numbers = [
    flag.get_attrib_text("NAME")
    for flag in flag_refs
    if flag.has_attrib("NAME")
]

print(flag_numbers)

Exploding Block References

This is an advanced feature and because ezdxf is still not a CAD application, the results may no be perfect. Non uniform scaling lead to incorrect results for text entities (TEXT, MTEXT, ATTRIB) and some other entities like HATCH with arc or ellipse path segments.

By default the “exploded” entities are added to the same layout as the block reference is located.

for flag_ref in msp.query('INSERT[name=="FLAG"]'):
    flag_ref.explode()

Examine Entities of Block References

If you just want to examine the entities of a block reference use the virtual_entities() method. This methods yields “virtual” entities with attributes identical to “exploded” entities but they are not stored in the entity database, have no handle and are not assigned to any layout.

for flag_ref in msp.query('INSERT[name=="FLAG"]'):
    for entity in flag_ref.virtual_entities():
        if entity.dxftype() == "LWPOLYLINE":
            print(f"Found {str(entity)}.")