On the Yin and Yang of French Cities

What your teacher said is basically correct, but the expression is not accurate. It should be said that there is generally no definite article in front of French city proper nouns, such as J'ai visité Paris, but there are still yin and yang, usually masculine, so: Paris est beau. We often use another expression: la ville de Paris est belle. In this way, some people often use negative slogans.

In addition, some definite articles are included in city proper nouns. Pay attention to the capitalization of this definite article when writing, such as: La Rochelle, Le Havre, La Haye. Then the yin and yang at this time will be determined according to this definite article, such as: La Rochelle est belle.

I hope my answer can satisfy your question.