node_modules/flask/sass/font/_font-fallback.function.scss

// Font Fallback Function
//
// Returns the fallback for a font.
// Prioritizes a defined fallback over a global/shared fallback.

@function font-fallback($family) {

  $family-key: font-family-key($family);

  @if (map-get(map-get($fonts, $family-key), fallback)) {
    @return map-get(map-get($fonts, $family-key), fallback);
  }
  @else if (map-get($font-fallbacks, font-type($family-key))) {
    @return map-get($font-fallbacks, font-type($family-key));
  }
}