Serve Current Directory over HTTP

The following command:

python -m SimpleHTTPServer 8181

will serve the current directory where the command is executed by starting a HTTP server on port 8181.

Reverse List

The wasListReverse will reverse a list containing elements of any type.

###########################################################################
##  Copyright (C) Wizardry and Steamworks 2016 - License: GNU GPLv3      ##
###########################################################################
def wasListReverse (l):
    if len(l) <= 1:
        return l
    return listReverse(l[1:]) + [ l[0] ]

Example call:

i = ["sdf", 11, 22, 39]
print listReverse(i)

Output:

[39, 22, 11, 'sdf']

Derivations

Using the programming language's features, we can derive different forms of wasListReverse.

Using the Ternary Operator

The listReverseTern function makes use of the ternary operator (introduced in Python 2.5) in order to get rid of the conditional in the wasListReverse function:

###########################################################################
##  Copyright (C) Wizardry and Steamworks 2016 - License: GNU GPLv3      ##
###########################################################################
def listReverseTern (l):
    return l if len(l) <= 1 else listReverseTern(l[1:]) + [ l[0] ]

Using Lambda Expressions

The listReverse lambda expression will leverage the ternary operator from the listReverseTern function and trim down the function to a single line:

###########################################################################
##  Copyright (C) Wizardry and Steamworks 2016 - License: GNU GPLv3      ##
###########################################################################
 
wasListReverseLambda = lambda x : x if len(x) <= 1 else wasListReverseLambda(x[1:]) + [ x[0] ]

Example call:

wasListReverseLambda = lambda x : x if len(x) <= 1 else wasListReverseLambda(x[1:]) + [ x[0] ]
 
print wasListReverseLambda(["sdf", 11, 22, 39])

Output:

[39, 22, 11, 'sdf']

Y-Combinator

By expressing the $Y$ combinator using the $\omega$ combinator as explained, the $Y$ combinator can be implemented in Python.

#!/usr/bin/python
###########################################################################
##  Copyright (C) Wizardry and Steamworks 2016 - License: GNU GPLv3      ##
###########################################################################
 
w = lambda x : x( x );
Y = lambda y : w( lambda f : y( lambda x : w(f)(x) ) );

An example call, to calculate the factorial of $10$ , would be:

Factorial = Y( lambda f : lambda n : 1 if n <= 1 else n * f(n - 1) );
print Factorial(10);

Only in (Monty) Python

TypeError: dump() takes at least 2 arguments (2 given)

Memory Error When Installing Packages with Pip

In case packages cannot be installed and a MemoryError exception is thrown, then most likely it is due to pip attempting to load packages in memory. To solve the issue, run pip with the additional –no-cache-dir parameter.

Installing Python Applications without Breaking the Environment

Linux distributions that contain python packages as part of the distribution packaging systems cannot really use pip to install packages due to the files installed by pip conflicting with the files from the distribution package. Typically this results in an error along the lines of "error: externally-managed-environment" or "This environment is externally managed" that suggests the user to add the –break-system-packages flag if the user wants to proceed anyway.

An alternative is to use pipx:

aptitude install pipx

and then to install the package using pipx:

pipx install unmanic

where:

unmanic is the package to be installed

pipx creates a separate virtual environment containing python along with all the requirements for the package to be installed thereby preventing conflicts with distribution files.

One trick is to specify PIPX_HOME and PIPX_BIN_DIR environment variables in order to make pipx install the application to a specific directory that will become the application directory instead of the application being installed in the same virtual environment as all other applications installed with pipx.

For example, the commands:

export PIPX_HOME=/opt/unmanic
export PIPX_BIN_DIR=/opt/unmanic/bin
pipx install unmanic

will install the unmanic application by creating a virtual environment starting from the filesystem path /opt/unmanic and by placing the unmanic binary in /opt/unmanic/bin.

Now, in order to install a different application, simply change the PIPX_HOME and PIPX_BIN_DIR environment variables.

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