This film is described as
The Plot: A romantic drama set in 1920s Paris, where the son of a courtesan (Friend) retreats into a fantasy world after being forced to end his relationship with the older woman (Pfeiffer) who educated him in the ways of love.
It clearly does not take place in the 1920's.From the look of the costume design it is 20 or 30 years earlier.
I don't know where you got that description but it says multiple times in the film that it's set during the Belle Époque (late 19th c/early 20th).
http://saucybetty.blogspot.com
Actually, the description of "1920s Paris" is from the title page right here at IMDb! They should check on who writes up those plot descriptions. Yes, the movie states quite clearly that it's set around the turn of the last century.
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