Getting useful info from QFontDatabase

I'll get back to the Last(goddam)Resort font issue shortly. But in learning about it I've had to look a little closer at what Qt knows about fonts, and as a result I've got a function that might be useful to others.

The QFontDatabase object contains whatever Qt knows about fonts that are "available in the underlying window system". However it makes this information available in a rather awkward way. You can call its families() method to get a list of all font names:

from PyQt5.QtWidgets import QApplication
from PyQt5.QtGui import QFont, QFontDatabase
the_app = QApplication([])
f_db = QFontDatabase()
for family in f_db.families():
    print(family)

My macbook is rather over-endowed with fonts, and this prints a list of 238 names.

You can also ask for the recommended or default fixed and "general" fonts,

qf_fixed = f_db.systemFont(QFontDatabase.FixedFont)
print( 'Fixed font:',qf_fixed.family() )
qf_general = f_db.systemFont(QFontDatabase.GeneralFont)
print( 'General font:',qf_general.family() )

Which on my macbook prints

Fixed font: Monaco
General font: .Lucida Grande UI

Here's a surprise: what the heck is that leading dot doing in the name of the general font? There is definitely no font named ".Lucida Grande" in the list of families, nor displayed by the Fontbook app, nor (using find /System ".Lucida*') in the System folder. However, the font db will create a QFont for it:

>>> lgf = f_db.font('.Lucida Grande UI','',12)
>>> lgf.family()
'.Lucida Grande UI'
>>> lgf.pointSize()
13

So it's real in some sense, and your code can use it. Moving on...

You can ask the font db things about any single family name, for example isFixedPitch(family). But font family names aren't consistent from system to system. And often what you want is a font that meets certain criteria, such as: it's "Times" by some name, or it has a bold or italic variant, or it scales to 36pt. So I put together this little function that will return a (possibly empty) list of fonts that meet given criteria:

def font_filter( name = None, style = None, fixed = False, sizes = [], language = None ) :
    db = QFontDatabase()
    selection = db.families()
    if name :
        selection = [family for family in selection if name in family ]
    if style :
        selection = [family for family in selection if style in db.styles(family) ]
    if fixed :
        selection = [family for family in selection if db.isFixedPitch(family) ]
    if language :
        selection = [family for family in selection if language in db.writingSystems(family) ]
    if sizes :
        size_set = set(sizes)
        selection = [family for family in selection
                     if size_set.issubset( set( db.smoothSizes(family, '' ) ) ) ]
    return selection

For example,

>>> print( font_filter( name = 'Helvetica' ) )
['Helvetica', 'Helvetica CY', 'Helvetica Neue']
>>> print( font_filter( fixed = True ) )
['Anonymous Pro', 'Courier', 'Courier New', 'Inconsolata', 'PCMyungjo', 'PT Mono']
>>> print( font_filter(fixed = True, style='Bold', sizes=[12, 18]) )
['Courier', 'Courier New', 'PT Mono']
>>> font_filter(fixed=True, language=QFontDatabase.Hebrew)
['Courier New']

Next time: back to the issue of the Last(goddam)Resort font problem.