Meaning of "by profession"

Dear teachers,

Please read:

They’re both doctors by profession.

Coul you explain me the meaning of “by profession”? what “by” here mean?Why there isn’t an article in front of “profession”?“by profession” is an expression?

Tung Quoc

By profession is commonly used and means

What are you by profession?(What’s your profession?)
I’m an engineer

The same:

What are you?
I’m an engineer

Hi Quoc

Yes, “by profession” is a fixed expression but it is usually not necessary to use it.

A: What does he do for a living?
B: He’s a doctor by profession.

A: What does he do?
B: He’s a doctor.