Currency converter - task from kiwi

version 1.0

Program for conversion from one currency to another

Featuring

• smart internet connection and currency conversion sites response detection
  • currency conversion sites implemented: fixer.io, currencylayer.com
• offline currencies sql database on disk
  • automatic offline database update on detection of new rates update from any implemented site
    • data_valid_utc is stored separately in txt file - faster then another sql database query
• time effective conversion
  • tested on windows 64
    • w/o update offline database triggered : ~ 50 ms
    • else : ~ 1200ms
• standard logger for logging
• argparser for input argument parsing - help string autogeneration
• manually tested
  • 3 types input arguments from assignment https://gist.github.com/MichalCab/3c94130adf9ec0c486dfca8d0f01d794
  • another common currency symbols - Kč, $, CNY-symbol, GBP-symbol, EUR-symbol
• used modules:
  • pandas, sqlalchemy, redis, redisworks, requests, json
  • arrow, argparse
  • platform, socket, sys, time, pathlib, logging, pickle
• using decorators: @staticmethod, @classmethod, @property
• docstrings documentation for all used functions (i hope), nearly PEP8 compatible

Usage

Parameters

• --amount - amount which we want to convert - float
• --input_currency - input currency - 3 letters name or currency symbol
• --output_currency - requested/output currency - 3 letters name or currency symbol

OUTPUT

• json with following structure:

{
   "input": {
        "amount": <float>,
        "currency": <3 letter currency code>
    }
    "output": {
        <3 letter currency code>: <float>
    }
}

Trigger online update

• to trigger offline sql disk database update from online cconversion sites
  • Declare global variable DEBUG with value including 'simulate_time'
    • This makes the [get_sites_states] function thing the time is 12 days in the future
    • Future date triggers the program to think new rates are available
  • Delete one or both files
    • db/rates_info.nfo = sql database valid_to date info
    • db/exchange_rates.db = sql database with offline rates

Disclaimer

Not using these controversial tricks to get lower latency

[x] Immediate exchange rate json output after reading (from sql/redis)

• implemented style: First the program tries to find out if the database should be updated
• not implemented: First run the program returns json data from offline database
  • even if in second thread updating the database afterwards
  • user gets better response time, but the first time he calls, he can get outdated rates
    • we certainly do not want that on the first start after a long time (olddated sql database)
    • it can be implemented w/o further ado, but only if the server with redis is updated frequently

[x] fixer.io database update

• as I was naive and trusted something written on the internet page, I did not implement online rates database update retardation
• for example now (09-04-2017 Sunday) the latest online fixer.io rates database is from 07-04-2017, even they state it is updated daily around 15:00 CET
• so if you encaunter the offline database is never used, it can be due to the fact the free currency sites update times can be sometimes missleading

Not implemented (yet) - future releases maybe

• full redis support for rates
  • expire time - to the known time of next update from any implemented site
• automatic selection of cconversion site dependent on offered currency codes conversion
  • e.g if CZK is not in the most fresh currency converter site - it will not be converted to/from
• currency conversion rates from multiple sources in the offline database
  • e.g. if CZK is not in the selected most fresh currency converter site - it will not be converted to/from
• generalization of currency conversion site to the point they can be generated by simple textual config
  • common CurrencyConverterSite class inheritance for every new conversion site
  • (working on it - almost functional - git branch common_ccs_class)
• multithreading - load from database at start in different thread - use the data or not later
  • after finding the offline data are not fresh enough
• CurrencySymbolConverter loading currency symbols conversions from a text file on disk (possibly slower)
• automatic database update as a background service
  • would update sql database and redis automatically not needed to check every time the convert is triggered
• automatic testing via asserts and node2 module
• online conversion site rates database update retardation detection
  • e.g. detect when "daily" updated site is not updated for 3 days - do not try to contact it every time for the old data completely ignoring the valid data at the offline sql database
• user input error autocorrection
  • suggestion of the textually nearest currency
  • another program argument for automatic nearest suggestion conversion
  • localisation of output / logger texts into other languages
• modules to possibly use:
  • hiredis - can be 10 times faster on windows..
  • BeautifulSoup - for automatic currency symbol to currency code conversion from site http://www.xe.com/symbols.php