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Brandon Rozek 2020-10-11 21:56:53 -04:00
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---
title: "Manage Python Applications"
date: 2020-10-11T19:21:10-04:00
draft: false
tags: []
---
I've recently discovered an application called [`pipx`](https://pipxproject.github.io/pipx/) which allows one to install and run Python applications in isolated environments. I think of it as a package similar to `apt`, but keeps the packages nice and isolated from one another.
To Install:
```bash
sudo apt install python3-venv pipx
pipx ensurepath
```
By default, it will create the virtualenvs in `~/.local/pipx` and drop executables in `~/.local/bin`.
Install `diceware` using `pipx`:
```bash
pipx install diceware
```
List the virtual environments maintained by `pipx`:
```bash
pipx list
```
Upgrade a package:
```bash
pipx upgrade diceware
```
Add additional dependencies into a package's virtual environment:
```bash
pipx inject package dependency
```

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title: "Tail Call Optimization in Python"
date: 2020-10-11T20:46:54-04:00
draft: false
tags: []
---
In a standard recursive function, a new stack frame is created every time a recursive call is made. This can lead to bad memory performance and as a protective measure, some programming languages have a maximum stack frame limit.
The following example is in Python:
```python
def factorial(n):
if n == 0:
return 1
return n * factorial(n-1)
```
```python
>>> factorial(1000)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 4, in factorial
File "<stdin>", line 4, in factorial
File "<stdin>", line 4, in factorial
[Previous line repeated 995 more times]
File "<stdin>", line 2, in factorial
RecursionError: maximum recursion depth exceeded in comparison
```
## Implementation
To get around this in Python, we need some sort of [trampoline](https://en.wikipedia.org/wiki/Tail_call#Through_trampolining), or way to exit part the stack and call the recursive function again. This technique is called Tail Call Optimizatoin, and we can do this in Python using exceptions. This code is heavily inspired by Crutcher Dunnavant's [code snippet](https://code.activestate.com/recipes/474088-tail-call-optimization-decorator/) from 2006.
```python
"""
Python Decorator that enables tail call
optimization through Exception Trampolines.
"""
from dataclasses import dataclass, field
from typing import Any, Dict, List
import functools
import inspect
@dataclass
class TailRecurseException(Exception):
"""Exception to enable tail call recursion."""
args: List[Any] = field(default_factory=list)
kwargs: Dict[Any, Any] = field(default_factory=dict)
def tail_call(func):
"""
Decorator that performs tail call optimization.
Notes
=====
Works by throwing an exception to exit the stack
when the function sees itself as its grandparent in
the stack trace. It then calls itself with its new
arguments.
"""
@functools.wraps(func)
def recurse(*args, **kwargs):
frame = inspect.currentframe()
if frame.f_back is not None \
and frame.f_back.f_back is not None \
and frame.f_back.f_back.f_code == frame.f_code:
raise TailRecurseException(args, kwargs)
while True:
try:
return func(*args, **kwargs)
except TailRecurseException as exception:
args = exception.args
kwargs = exception.kwargs
return recurse
```
## Example: Factorial
Now let's redefine `factorial` in a tail-call way:
```python
@tail_call
def factorial(n, acc=1):
if n == 0:
return acc
return factorial(n - 1, n * acc)
```
For the moment of truth...
```python
>>> factorial(1000)
402387260077093773543702433923003985719374864210714632543799910429938512398629020592044208486969404800479988610197196058631666872994808558901323829669944590997424504087073759918823627727188732519779505950995276120874975462497043601418278094646496291056393887437886487337119181045825783647849977012476632889835955735432513185323958463075557409114262417474349347553428646576611667797396668820291207379143853719588249808126867838374559731746136085379534524221586593201928090878297308431392844403281231558611036976801357304216168747609675871348312025478589320767169132448426236131412508780208000261683151027341827977704784635868170164365024153691398281264810213092761244896359928705114964975419909342221566832572080821333186116811553615836546984046708975602900950537616475847728421889679646244945160765353408198901385442487984959953319101723355556602139450399736280750137837615307127761926849034352625200015888535147331611702103968175921510907788019393178114194545257223865541461062892187960223838971476088506276862967146674697562911234082439208160153780889893964518263243671616762179168909779911903754031274622289988005195444414282012187361745992642956581746628302955570299024324153181617210465832036786906117260158783520751516284225540265170483304226143974286933061690897968482590125458327168226458066526769958652682272807075781391858178889652208164348344825993266043367660176999612831860788386150279465955131156552036093988180612138558600301435694527224206344631797460594682573103790084024432438465657245014402821885252470935190620929023136493273497565513958720559654228749774011413346962715422845862377387538230483865688976461927383814900140767310446640259899490222221765904339901886018566526485061799702356193897017860040811889729918311021171229845901641921068884387121855646124960798722908519296819372388642614839657382291123125024186649353143970137428531926649875337218940694281434118520158014123344828015051399694290153483077644569099073152433278288269864602789864321139083506217095002597389863554277196742822248757586765752344220207573630569498825087968928162753848863396909959826280956121450994871701244516461260379029309120889086942028510640182154399457156805941872748998094254742173582401063677404595741785160829230135358081840096996372524230560855903700624271243416909004153690105933983835777939410970027753472000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
```
## Example: Fibonacci Numbers
```python
@tail_call
def fib(n, a=0, b=1):
if n == 0:
return a
if n == 1:
return b
return fib(n - 1, b, a + b)
```
```python
>>> fib(1000)
43466557686937456435688527675040625802564660517371780402481729089536555417949051890403879840079255169295922593080322634775209689623239873322471161642996440906533187938298969649928516003704476137795166849228875
```
## Conclusion
You can install the package via pypi:
```bash
pip install tail-recurse
```
It is also available on [Github](https://github.com/Brandon-Rozek/tail-recurse/).