pyLife’s variable name conventions

Preamble

In order for source code to be readable and maintainable, variable names should be expressive, i.e. they should imply what the variable represents. By doing that, documenting variable names becomes virtually unnecessary.

However, in scientific programming we often need to deal with fairly complex mathematical equations. Then it is tempting to use the same or at least similar symbols as we find in the equation in the text book. While these symbols are obvious to people with domain knowledge, for programmers focusing on software optimization and industrialization these symbols are often hard to read.

Out of these considerations we decided that in pyLife for physical quantities the variable names as described in this document are mandatory. For physical quantities not described in this document, you can either use an expressive variable name or you can document a symbol in your module documentation.

General rules

Letters

Roman letters can be used as is, capital or small. Greek letters could actually also be written as unicode letters. Yes, x = x_0 * np.exp(-δ*t) * np.cos(ω*t + φ) is perfectly valid Python code. However, not all of us are using decent systems which allow you to type them easily. That’s why for Greek letters we would spell them out like alpha. This does not work for lambda, though as it is a keyword in python.

Indices

Indices should be separated with an underscore (_) from the symbol. However, in some cases the underscore is not uses (see below.)

Variable names

Stress values

Stress tensor variables: S11, S22, S33, S12, S13, S23
Cyclic stress variables: * amplitude: stress or load amplitude, * meanstress: meanstress, * R: R-value

Coordinate values

Cartesian coordinates: x, y, z

Displacement values

Displacements in a Cartesian coordinate system: dx, dy, dz

Strength variables

SD endurance limit in load direction, SD_xx for xx percent failure probability
ND endurance limit in cycle direction, ND_xx for xx percent failure probability
TS scatter in load direction (= SD_10/SD_90)
TN scatter in load direction (= ND_10/ND_90)
k slope of the Wöhler curve, k_1 above the endurance limit, k_2 below the endurance limit