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Use Jinja-like attribute substitution syntax #254

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merged 4 commits into from Apr 28, 2018

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ubruhin commented Apr 22, 2018

See motivation for this change in LibrePCB/librepcb-rfcs#14.

This PR changes the attribute substitution syntax from e.g. #NAME to {{NAME}}, which corresponds to the Jinja2 syntax.

Beside the disadvantage of the more verbose syntax (3 characters more), it has several advantages:

  • Escaping is very unlikely necessary (# may sometimes be used as raw character, but {{ probably not).
  • End of key is manually specified by }}, so it's always clear where the substitution ends. In the old syntax it was not possible to only substitute for example "FOO" in #FOObar. To work around that limitation, we defined || as a manual end, which looked very strange (#FOO||bar).
  • Fallbacks can be written very elegant. For example {{ FOO or BAR or 'foobar' }} evaluates to the value of FOO if it's non-empty. If it's empty, it evaluates to the value of BAR. If this is also empty, it evaluates to the literal "foobar". In the old syntax this was written as #FOO|BAR, while the fallback to literals was not supported. Note: This is actually not a Jinja2 feature, but part of the Python language.
  • If we need more functionality some time in future, Jinja2 provides much more functionality which we may support too. For example filters may be helpful in some use-cases.

Maybe this sounds a bit over engineered at this time, but I think it's better to switch now (before the first release) to a more powerful, future-oriented syntax. After publishing the first release of LibrePCB it would be a pain to change the attribute substitution syntax...

Btw, if you wonder what the fallback feature is useful for, in the Multiple Key Substitution documentation there is a real-world use case explained.

How the most important attributes NAME and VALUE now look in the symbol and footprint editors:

auswahl_011

auswahl_010

ubruhin added some commits Apr 19, 2018

Use Jinja-like syntax for attributes
- Old syntax: #KEY
- New syntax: {{KEY}}
Update submodule "demo-workspace"
Use Jinja-like syntax for attributes

@ubruhin ubruhin added this to the 0.1 milestone Apr 22, 2018

@ubruhin ubruhin self-assigned this Apr 22, 2018

@ubruhin ubruhin merged commit 70a745f into master Apr 28, 2018

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@ubruhin ubruhin deleted the attributes_jinja_syntax branch Apr 28, 2018

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