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A python package for planarity testing and rendering of ladder type graphs

Project description


Harald Ujc - Screenpop Software Inc.

A python library for planarity testing and rendering of ladder type graphs. Built with Visual Studio Code and Python 3.7.0 32-bit, 3.7.4 64-bit Conda on Windows 10 and Python 3.7.3 64-bit, 3.7.3 64-bit Conda on Mac OS X.


pyladder is an exercise in translating an ANSI 'C' program to a python class. The program will take a list of nodes representing a connected graph in the plane. It will then attempt to generate a visual display of the graph, and advise if the graph is planar or not.

Note that as this is a learning exercise, some of the style used will not adhere to generally accepted python patterns. There is an upper limit on the number of connections and nodes. This is not a limitation of the algorithm, rather due to the original 'C' program being a proof-of-concept and there was no appetite for dynamic memory management and fixed arrays were used instead.

Practical applications

Programmable logic controllers

The class can be used to display a ladder representing protective interlock logic.


The class can be used to render a maze or connected points in space in real time from a list of coordinates. This is much more portable, space saving and dynamic as opposed to a fixed graphical representation in files or memory.

Planarity testing

The input node list could represent a list of electronic parts and the output could then be used to create an circuit board etch where the requirement is that the connecting line segments cannot cross, for obvious reasons.

Calling and usage convention 1

A python dictionary describing the ladder as follows: The key-value pair is key = a string label identifying the node value = a list of nodes to which this code connects. The first element of each list is the nodes key, and the subsequent items are the keys of the nodes to which this node connects Example :

from pyladder import Pyladder
my_ladder = Pyladder()
ladder_input = {'Part 10' : [10,20,30,40,50], 'Part 20' : [20,30,40,50], 'Part 30' : [30,40], 'Part 40' : [40,50], 'Finish' : [50]}
if not my_ladder.display_graph_plot('Nodes', ladder_input):
  print('Ladder is not planar')

Note that it is up to the client to create the dictionary.

The above call to display_graph_plot will return true or false according to the ladder planarity and will display as per below:

VS Code in action

LINK17.DAT file will display as per below:

VS Code in action

LINK18.DAT is a non-planar ladder. A 'Ladder is not planar' message will be shown on the command line. The code will attempt to render the graph as much as possible without crossing connections (or line segments) although the correctness of the plot is not gauranteed. LINK18.DAT will display the following:

VS Code in action

Calling and usage convention 2

Two lists containing the ladder nodes as follows: The first list represents a node and its connections to other nodes i.e. [x, y, z, ...] where x is the subject node and y, z, ... are the nodes to which it connects This list is passed to the rendering engine

The second list is metadata about the first list, and is used only by matplotlib ['node description 1', 'node description 2', 'node description 3',...] where 'node description 1 mapes to 'x' in the first list Example:

import pyladder as pylad
my_ladder = pylad.pyladder()
ladder_input = [[10,20,30,40,50], [20,30,40,50],[30,40],[40,50],[50]]
graph_node_labels = ['Part A','Part B','Part C','Part D', 'Finish']

if my_ladder.gen_graph(ladder_input):
  coors = my_ladder.get_render()
  print('Ladder is not planar')

The call to gen_graph will return true or false according to the ladder planarity.

Here, coors is a list of coordinate pair lists representing the line segments to be plotted:

[ [[x1,y1], [x2,y2]], [[x3,y3], [x4,y4]], ..., [[xN,yN], [xM,yM]], ]

Calling and usage convention 3

A list containing coordinate pair lists each representing an edge connecting two nodes.


import pyladder as pylad

ladder_edge_list = [[10,20], [10,30], [10,40], [10,50], [20,30], [20,40], [20,50], [30,40], [40,50]]

my_ladder = Pyladder()

if my_ladder.display_graph_plot_edges('Nodes', 'dictionary input', ladder_edge_list):
    print("Ladder is planar")
    print("Ladder is not planar")

The call to display_graph_plot_edges will return true or false according to the ladder planarity.

Refer to,, and for usage examples

A note on the *.DAT files

The *DAT files were the input files to the original 'C' command line program. Format used is one node identifier per line. The first node is the 'source', and every node after up to '0' are the nodes to which the source connects.


10 20 25 40 0 20 39 0 ...

The sample client file can be used to injest and parse the *.DAT files into the dictionary data structure format required by, as described in calling convention 1 and the ladder_input dictionary specifically.

Non-planar samples

The following files are examples of non-planar ladders: LINK2.DAT LINK4.DAT LINK10.DAT LINK18.DAT

Issue log

1; The label for the top 'rung' is not displayed in the matplotlib line plot. Status = Fixed 2; The ladder representation in file LINK32.DAT is returning a 'graph rendering failed' message. Status = Fixed 3; Improve the visual rendering by including a marker on the vertical line segments for each level of the ladder. This would be helpful when using the class to render PLC ladder logic. Status = Oustanding 4; Comment out debug lines. Presently the command line output is very verbose. Status = Fixed 5; File LINK36.DAT is not rendering correctly (out of order along y-axis). Status = Fixed 6; File LINK20.DAT is not rendering correctly (out of order along y-axis). Status = Fixed 7; File LINK30.DAT is not rendering correctly. Status = Fixed, however the ladder is too large to display, must find a scrollable plotting tool. 8; Implement a scrollable visual plotting library. Status = Outstanding 9; Implement a non-visual method call that returns only a planar/non-planar boolean. Can be used for batch jobs. 10; Transition this issue log to github issues.

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