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36 lines
1.7 KiB
Markdown
36 lines
1.7 KiB
Markdown
# Doughnut — An exchange rate tracker for people nuts about dough, in PyQt.
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This is a simple currency exchange rate tracker implemented in PyQt, using the [fixer.io](http://fixer.io) API
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for data. The default setup shows currency data for the preceding 180 days.
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Data is loaded progressively, with increasing resolution. Currency rates for a given date are shown in the right
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hand panel and updated to follow the position of the mouse.
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> If you think this app is neat and want to learn more about
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PyQt in general, take a look at my [free PyQt tutorials](https://www.learnpyqt.com)
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which cover everything you need to know to start building your own applications with PyQt.
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## Code notes
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### Data handling
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The interface presents a tracking plot (using PyQtGraph) of rates over the past 180 days. Since we don't want to
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spam a free service, requests to the API are rate-limited to 1-per-second, giving a full-data-load time of 180s (3 min).
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To avoid waiting each time, we use `requests_cache` which uses a local sqlite database to store the result of recent
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requests. The requests for data use a progressive 'search' approach: where there is a gap in the data, the middle
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point is filled first, and it prefers to load the most recent timepoints first. This means the whole plot gradually
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increases in resolution over time, rather than working backwards only.
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### Conversions
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By default the app retrieves EUR rates and shows conversions to this base currency. If you change base currency
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it will retrieve all data again for that new currency. This is daft, since if we have rates vs. EUR we can calculate
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any other currency->currency conversion via EUR (with a small loss of accuracy).
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