generate_netlist(): Use default footprint for part if none is defined

Is your feature request related to a problem? Please describe. It can be a pain to find the footprint for parts so it would be nice to use the default footprint for generate_netlist() if the user doesn’t define one when creating the part. Also, the code is smaller and easier to read without a footprint defined.

Describe the solution you’d like generate_netlist() should use the default footprint for a part if none is provided when the user creates the part

[devbisme]

I pushed a commit to the development branch with the empty_footprint_handler() function. Here’s an example of how to use it:

import skidl

def my_empty_footprint_handler(part):
    """Function for handling parts with no footprint.

    Args:
        part (Part): Part with no footprint.
    """
    ref_prefix = part.ref_prefix.upper()

    if ref_prefix in ("R", "C", "L") and len(part.pins) == 2:
        # Resistor, capacitors, inductors default to 0805 SMD footprint.
        part.footprint = 'Resistor_SMD:R_0805_2012Metric'

    elif ref_prefix in ('Q',) and len(part.pins) == 3:
        # Transistors default to SOT-23 footprint.
        part.footprint = 'Package_TO_SOT_SMD:SOT-23'

    else:
        # Everything else goes to the default empty footprint handler.
        skidl.default_empty_footprint_handler(part)

# Replace the default empty footprint handler with your own handler.
skidl.empty_footprint_handler = my_empty_footprint_handler

# Create parts with no footprints.
r = skidl.PartTmplt("Device", "R")
r1, r2 = r(), r()
r2.footprint='Resistor_SMD:R_1206_3216Metric'

# Generate a netlist. R1 has no footprint so it will be assigned an 0805.
# R2 already has a footprint so it will not change because it is not passed
# to the empty footprint handler.
skidl.generate_netlist()

[devbisme]

Maybe the thing to do is define a set_empty_footprint(part) function that gets called during netlist generation if the footprint is empty. Normally, the function will raise an exception, but the user can redefine it so that it substitutes a default footprint for a given type of part and raises an exception for part types it doesn’t handle.