Rob Siegwart - Python

Python @staticmethod, @classmethod

2021-06-09T00:00:00-05:00

While regular class methods take in the class instance as the first parameter, @classmethod and @staticmethod are available in Python to create class methods with different behavior and usage.

@staticmethod

does not take in the class or the instance as the implicit first parameter
can be called on either the class or an instance
cannot access class or instance properties

A typical usage might be to store any general utility function to the class, such as when the arguments have nothing to do with the class or its instances.

class Foo:
    @staticmethod
    def add(a,b):
        return a + b

Foo.add(3,12)

@classmethod

takes the class in as the implicit first parameter
can be called on either the class or an instance
has access to class properties

A typical usage might be to create new class instances with some alternative call signature:

class Foo:
    def __init__(self, a, b):
        self.a = a
        self.b = b

    @classmethod
    def bar(cls, a, c):
        return cls(a, c**2)

standard = Foo(2,9)
alternate = Foo.bar(2,3)

Easily Zip Ghost Themes with Python

2021-06-01T00:00:00-05:00

Previous iterations of this site used the Ghost blogging platform which utilized a custom theme. In order to upload it to the Ghost application a zipped folder of the theme is required. To save mouse clicks and keyboard strokes, I wrote a Python script to zip the theme files and save out a .zip file with the version included in the filename.

For Ghost, the theme version is set in the package.json and package-lock.json files for it to be recognized in the Ghost admin. The Python script below gets the version value from package.json and includes it in the zipped file name.

The Python script is placed in the directory to be zipped and some variables are set:

BASE_NAME - base name of the output zip file with the output name being [BASE_NAME]-[VERSION].zip
OUTDIR - local output directory name for the resulting zip file
REMOVE_DIRS and REMOVE_FILES - lists of directories and files, respectively, to exclude in the resulting zip file

Then, running this file compiles the files in the current directory and all subdirectories with the exceptions listed in REMOVE_DIRS and REMOVE_FILES and places an named zip file in the directory specified in OUTDIR.

import os
import glob
import zipfile
import json

BASE_NAME = 'my_zip_file'
OUTDIR = 'Zipped'
REMOVE_DIRS = ['Zipped','.vscode','.git']
REMOVE_FILES = ['VERSION','zip.py','routes.yaml']

with open('package.json') as f:
    packagejson = json.load(f)
    version = packagejson['version']

theme_files = set(glob.glob('*'))
theme_files = theme_files - set(REMOVE_DIRS) - set(REMOVE_FILES)

if not os.path.exists(OUTDIR):
    os.mkdir(OUTDIR)

zip = zipfile.ZipFile(
    os.path.join(OUTDIR, f'{BASE_NAME}-{version}.zip'),
    mode='w',
    compression=zipfile.ZIP_DEFLATED
)

def zipdir(directory):
    for root, dirs, files in os.walk(directory):
        for file in files:
            zip.write(os.path.join(root, file))
        for dir in dirs:
            zipdir(dir)

for file in theme_files:
    if os.path.isdir(file):
        zipdir(file)
    else:
        zip.write(file)

zip.close()

matplotlib Quick Ref

2021-04-03T00:00:00-05:00

Preliminaries

To import matplotlib, the recommended way is:

import matplotlib.pyplot as plt

Basic Plotting with plt

The imported plt can be used for interactive and basic usage, similar to MATLAB. A typical workflow might be:

Assemble your data

Create a plot using one of the plot types

Add options, legends, axes configurations, annotations using plt.* methods

Call plt.show() to show the plot (may or may not be needed depending on the environment/software you are using)

See here for the full list of methods and API.

Some Common Methods

Plot types

plt.plot
plt.bar
plt.pie

Annotations

plt.axhline, plt.axvline
plt.arrow
plt.annotate, plt.text
plt.title

Axis

plt.xlabel, plt.ylabel
plt.xticks, plt.yticks
plt.xlim, plt.ylim

Object Approach

This approach is initiated by calling the subplots() function with returns separated Figure and Axes objects, allowing direct access to each respective object:

fig, ax = plt.subplots()

This function also allows separate figures and multi-axis figures to be created and assigned to separate variables:

fig1, ax1 = plt.subplots()
fig2, ax2 = plt.subplots()

Additionally, multiple axes (i.e. plots) in one figure can be created with ncols and nrows:

fig, (ax1, ax2) = plt.subplots(nrows=1, ncols=2)

Use the sharey and sharex arguments to share x or y axes between subplots:

fig, (ax1, ax2) = plt.subplots(nrows=1, ncols=2, sharey=True)

Plot calls may then be used on the Axes object:

ax1.plot(x,y)

To save the plot to a file:

fig1.savefig('my_figure.png')

To show the plot, call plt.show() or fig1.show() - which one is preferable depends on the environment and context you are using as explained here2. Also, depending on your environment these may not not be needed at all (such as in iPython/Jupyter).

Configuring

Color, linestyle, linewidth, and other properties of the plot can be specified in the plot call.

See here3 for a full list of options.

Options

Property	Keyword argument
Line color	`color` or `c`
Line width	`linewidth` or `lw`
Line style	`linestyle` or `ls`
Marker style	`marker`

Colors

Colors may be entered in a variety of ways - string name, RGB tuple, HEX string, etc. A range of color names are included in matplotlib by default. Basic colors are available in addition to those named in the X window system and the xkcd color survey.

Basic colors include: blue/ b, green/ g, red/r, cyan/ c, magenta/ m, yellow/ y, black/ k, white/ w.

Legend

Add a legend by calling ax.legend(). For the legend to have names, labels are typically first specified in the plot call with the legend keyword argument:

ax1.plot(t, v, legend='Velocity')

Alternatively, if you store a reference or get the line object from an existing axes, you can call:

line.set_label(str)

Then, when calling ax.legend() the legend generator knows names for each line. These methods are preferred since the names and data are linked together.

Placement of the legend can be set to 9 points forming a grid pattern over the axes, by setting the loc argument to a combination of upper, lower, left, right, center (for example upper left). center only places it in the middle of both directions. best places the legend in the area with the minimum amount of overlap with other elements.

See the API for the many other available options.

Other Things

Grids

Turning on grids can be achieved with:

ax.grid(True, which='both')

The which argument specifies which axis to turn them on for (x, y, or both).

Styles

There are some predefined styles to use in your plots. The style names for these include:

Solarize_Light2
_classic_test_patch
bmh
classic
dark_background
fast
fivethirtyeight
ggplot
grayscale
seaborn
seaborn-bright
seaborn-colorblind
seaborn-dark
seaborn-dark-palette
seaborn-darkgrid
seaborn-deep
seaborn-muted
seaborn-notebook
seaborn-paper
seaborn-pastel
seaborn-poster
seaborn-talk
seaborn-ticks
seaborn-white
seaborn-whitegrid
tableau-colorblind10

A style can be turned in your session via:

plt.style.use(<name>)

To activate a style for just a specific plot, you can use the with mechanism:

with plt.style.context(<name>):
    plt.plot(x,y)

Or,

with plt.style.context(<name>):
    fig,ax = plt.subplots()
    ax.plot(x,y)

Square/1:1 plots

Use Axes.set_aspect('equal') to make the axes scales have a 1:1 ratio.

reStructuredText Elements

2020-10-26T00:00:00-05:00

reStructuredText is a great markup language used in the Python world but can also be used for general text activities. It has more specificity in its design and has more functionality when compared to others in its class such as Markdown.

It is the format for content for this website since I use the Pelican static site generator which natively uses RST format for writing content.

The below is a side-by-side comparison of most of the reStructuredText elements and their equivalent when processed; in this case pandoc was used to convert from RST to html. This list also does not include all forms of each element - for the complete specification see the link above.

inline markup
paragraph
bullet list
enumerated list
definition list
field list
literal block
line block
block quote
table (simple)
table (grid)
footnotes
citation
hyperlink (internal)
hyperlink (external)
hyperlink (embedded)
directive substitution

reStructuredText Result

inline markup

*emphasis*

**strong emphasis**

`interpreted text`

``inline literal text``

emphasis

strong emphasis

interpreted text

inline literal text

paragraph

Lorem ipsum dolor sit amet, consectetur
adipiscing elit. Pellentesque ornare
commodo est eget tempor. Pellentesque
vitae nulla velit.

Praesent non nisl id eros gravida
tempor eget in sapien. Curabitur
vestibulum dapibus placerat.

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque ornare commodo est eget tempor. Pellentesque vitae nulla velit.

Praesent non nisl id eros gravida tempor eget in sapien. Curabitur vestibulum dapibus placerat.

bullet list

- Lorem ipsum dolor sit amet
- Pellentesque ornare
- Pellentesque vitae nulla
  gravida tempor eget in sapien
- Nunc euismod tellus turpis,
  efficitur non.

Lorem ipsum dolor sit amet
Pellentesque ornare
Pellentesque vitae nulla gravida tempor eget in sapien
Nunc euismod tellus turpis, efficitur non.

enumerated list

A. Lorem ipsum dolor sit amet,
   consectetur adipiscing elit.
B. Pellentesque ornare commodo
C. Pellentesque vitae nulla
   gravida tempor eget in sapien

a. Lorem ipsum dolor sit amet,
   consectetur adipiscing elit.
b. Pellentesque ornare commodo

  i. Lorem ipsum dolor sit amet,
      consectetur adipiscing elit.
  ii. Pellentesque ornare commodo
      est eget tempor.

Lorem ipsum dolor sit amet, consectetur adipiscing elit.
Pellentesque ornare commodo
Pellentesque vitae nulla gravida tempor eget in sapien

Lorem ipsum dolor sit amet, consectetur adipiscing elit.
Pellentesque ornare commodo
1. Lorem ipsum dolor sit amet, consectetur adipiscing elit.
2. Pellentesque ornare commodo est eget tempor.

definition list

Lorem ipsum
    dolor sit amet

    consectetur adipiscing elit.

Pellentesque
    ornare commodo est eget tempor.

Lorem ipsum

dolor sit amet

consectetur adipiscing elit.

Pellentesque

ornare commodo est eget tempor.

field list

:Lorem ipsum:
    dolor sit amet

    consectetur adipiscing elit.

:Pellentesque:
    ornare commodo est eget tempor.

:Vestibulum:
    - vitae arcu ex
    - Fusce aliquet faucibus rutrum
    - Integer at tortor tellus

Lorem ipsum

dolor sit amet

consectetur adipiscing elit.

Pellentesque

ornare commodo est eget tempor.

Vestibulum

vitae arcu ex
Fusce aliquet faucibus rutrum
Integer at tortor tellus

literal block

Duis imperdiet id orci quis consequat.
Nam in sem consequat, laoreet enim ut,
aliquet nibh. Sed vitae sem ipsum.
Aliquam nec gravida dolor. ::

    Morbi mollis vitae mauris ac interdum.
    Pellentesque id posuere magna, fermentum
    Nam libero lacus, elementum eu faucibus
    aliquet, venenatis quis augue.

    Quisque rutrum arcu elit, sed tempor
    sed. Donec bibendum sit amet libero

Duis imperdiet id orci quis consequat. Nam in sem consequat, laoreet enim ut, aliquet nibh. Sed vitae sem ipsum. Aliquam nec gravida dolor. :

Morbi mollis vitae mauris ac interdum.
Pellentesque id posuere magna, fermentum
Nam libero lacus, elementum eu faucibus
aliquet, venenatis quis augue.

Quisque rutrum arcu elit, sed tempor
sed. Donec bibendum sit amet libero

line block

| Duis imperdiet id orci quis consequat.
| Nam in sem consequat, laoreet enim ut
|
| Sed vitae sem ipsum. Aliquam nec
| mauris ac interdum. Pellentesque
| fermentum dapibus purus.
| eu faucibus aliquet

Duis imperdiet id orci quis consequat.
Nam in sem consequat, laoreet enim ut

Sed vitae sem ipsum. Aliquam nec
mauris ac interdum. Pellentesque
fermentum dapibus purus.
eu faucibus aliquet

block quote

Duis imperdiet id orci quis consequat.
Nam in sem consequat, laoreet enim ut,

    Morbi mollis vitae mauris ac interdum.
    Pellentesque id posuere magna, fermentum dapibus purus.
    Nam libero lacus, elementum eu faucibus
    aliquet, venenatis quis augue.

    -- Morbi mollis

Duis imperdiet id orci quis consequat. Nam in sem consequat, laoreet enim ut,

Morbi mollis vitae mauris ac interdum. Pellentesque id posuere magna, fermentum dapibus purus. Nam libero lacus, elementum eu faucibus aliquet, venenatis quis augue.

-- Morbi mollis

table (simple)

================== =======================
Morbi mollis       vitae mauris
================== =======================
Pellentesque
id posuere magna   fermentum dapibus purus
Nam libero         aliquet, venenatis quis
================== =======================

Morbi mollis	vitae mauris
Pellentesque
id posuere magna	fermentum dapibus purus
Nam libero	aliquet, venenatis quis

table (grid)

+------------------+---------------------------+
| Morbi mollis     | vitae mauris              |
+==================+===========================+
| Pellentesque     |                           |
+------------------+---------------------------+
| id posuere magna | fermentum dapibus purus   |
+------------------+---------------------------+
| Nam libero       | aliquet, venenatis quis   |
+------------------+---------------------------+

Morbi mollis	vitae mauris
Pellentesque
id posuere magna,	fermentum dapibus purus
Nam libero	aliquet, venenatis quis

footnotes

Lorem ipsum dolor sit amet, consectetur
adipiscing elit. Pellentesque ornare
commodo est eget tempor. Pellentesque
vitae nulla velit [1]_.

Praesent non nisl id eros gravida
tempor eget in sapien. Curabitur
vestibulum dapibus placerat [2]_.


.. [1] Lorem ipsum dolor sit amet
.. [2] Duis imperdiet id orci quis consequat.

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque ornare commodo est eget tempor. Pellentesque vitae nulla velit¹.

Praesent non nisl id eros gravida tempor eget in sapien. Curabitur vestibulum dapibus placerat².

Lorem ipsum dolor sit amet↩︎
Duis imperdiet id orci quis consequat.↩︎

citations

Lorem ipsum dolor sit amet, consectetur
adipiscing elit. Pellentesque ornare
commodo est eget tempor. Pellentesque
vitae nulla velit [Lorem1]_.

.. [Lorem1] Lorem ipsum dolor sit amet

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque ornare commodo est eget tempor. Pellentesque vitae nulla velit [Lorem1].

Lorem1: Lorem ipsum dolor sit amet

hyperlink (internal)

Lorem ipsum dolor sit amet, consectetur
adipiscing elit. Pellentesque ornare
commodo est eget tempor. Pellentesque
vitae nulla velit.

.. _internal1:

The previous line specifies a hyperlink
target which links to this paragraph by
giving it an id.

Later on to link to this paragraph,
use internal1_.

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque ornare commodo est eget tempor. Pellentesque vitae nulla velit.

The previous line specifies a hyperlink target which links to this paragraph by giving it an id.

Later on to link to this paragraph, use internal1.

hyperlink (external)

Lorem ipsum dolor sit amet, consectetur
adipiscing elit. Pellentesque ornare
commodo est eget tempor. python_
pellentesque vitae nulla velit.

Praesent non nisl id eros gravida
tempor eget in sapien. Curabitur
vestibulum dapibus placerat
`asme home`_.

.. _python: https://python.org
.. _asme home: https://asme.org

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque ornare commodo est eget tempor. python pellentesque vitae nulla velit.

Praesent non nisl id eros gravida tempor eget in sapien. Curabitur vestibulum dapibus placerat asme home.

hyperlink (embedded)

Click `here <https://asme.org>`_ for the ASME
home page.

Click here for the ASME home page.

directive substitution

Lorem ipsum dolor sit amet, consectetur
adipiscing elit. |pellentesque| ornare
commodo est eget tempor. Pellentesque
vitae nulla velit.

Praesent non nisl id eros gravida
tempor eget in sapien. Curabitur
vestibulum dapibus placerat.

.. |pellentesque| math:: a^2 + xb^3

Lorem ipsum dolor sit amet, consectetur adipiscing elit.

a² + xb³

ornare commodo est eget tempor. Pellentesque vitae nulla velit.

Praesent non nisl id eros gravida tempor eget in sapien. Curabitur vestibulum dapibus placerat.

Merging and Watermarking PDF Files with Python

2019-08-25T00:00:00-05:00

There are many Python libraries available for working with PDF files. Here we will perform merging and watermarking operations with the Python library pdfrw. This package is bundled in the WinPython distribution.

It is assumed that your path has been set to find the correct Python interpreter such that pdfrw can be imported.

Merging

A merging script can be created in only a few lines of code. Let's create a script that will merge all of the PDF files within a directory, using a command line argument to specify the output file name. Locate the script in the same directory as the files, and it will grab all of the files with a .pdf extension and then combine those (in the same order as if issuing an 'ls' command) into a single output PDF file.

# merge.py

import sys
from glob import glob
from pdfrw import PdfReader, PdfWriter

def main():
    output_file = sys.argv[1]
    files = glob('*.pdf')
    writer = PdfWriter()
    for each in files:
        pdf = PdfReader(each)
        writer.addpages(pdf.pages)
    writer.write(output_file)

if __name__ == '__main__':
    main()

This can then be run with:

python merge.py <output file>

in the terminal or command prompt.

Watermarking

With watermarking, a separate pdf file (the watermark) is overlaid on top of another pdf file (the base file). For the watermark to show up at the correct location the watermark file must be the same size as the base pdf file. With this script the first argument will be the input file to apply the watermark to, and a second argument which is the name of the pdf file to use as the watermark. Again the script is to reside in the same directory as the files.

# watermark.py

import sys
from pdfrw import PdfReader, PdfWriter, PageMerge

def main():
    input_file = sys.argv[1]
    watermark_file = sys.argv[2]

    input_pdf = PdfReader(input_file)
    watermark = PdfReader(watermark_file).pages[0]

    for page in input_pdf.pages:
        merger = PageMerge(page)
        merger.add(watermark).render()

    writer = PdfWriter()
    writer.write(input_file.split('.pdf')[0]+'_w.pdf',input_pdf)

if __name__ == '__main__':
    main()

To run, in a terminal or command prompt issue:

python watermark.py <input file> <watermark file>