It's a brave question setter who asks about the actual work that Cabaret is based on.
Two short novels by Christopher Isherwood – Goodbye to Berlin and Mr. Norris Changes Trains – were published together in 1945, under the title The Berlin Stories. They are both set in Berlin in the early 1930s. Goodbye to Berlin included a short story entitled Sally Bowles, which had first been published in 1937.
The opening page of Goodbye to Berlin includes the line "I am a camera with its shutter open, quite passive, recording, not thinking." In 1951 the play I Am a Camera, written by the English playwright John Van Druten, opened on Broadway. Van Druten made Sally Bowles the central character in his play, which was filmed in 1955 – starring Julie Harris as Sally, and Laurence Harvey as Christopher Isherwood.
Depending on which page you read on Wikipedia, the play I Am a Camera was based either on The Berlin Stories (i.e. on both Goodbye to Berlin and Mr. Norris Changes Trains) or on Goodbye to Berlin alone.
The Kander and Ebb musical Cabaret opened on Broadway in 1966. It was based on both Goodbye to Berlin (the Isherwood novel) and I Am a Camera (the Van Druten play). The 1972 film version of Cabaret is, according to Wikipedia, "loosely based" on the Broadway musical.
So, to summarise: the film Cabaret (starring Liza Minnelli and Michael York) is loosely based on the Broadway musical of the same title, which was based on the play I Am a Camera by John Van Druten. I Am a Camera was based on Goodbye to Berlin and/or Mr. Norris Changes Trains (two short novels by Christopher Isherwood, which were published under the title The Berlin Stories). The central character in Cabaret first appeared as the title character of a short story that was first published in 1937, and included in Goodbye to Berlin. According to Wikipedia, Isherwood originally intended to include the story in Mr. Norris Changes Trains.
Including Sally Bowles and the original Broadway musical Cabaret, I make that six different works on which the 1972 film could be said to be at least partly based. They are all by Christopher Isherwood, except I Am a Camera which is by John Van Druten and the musical Cabaret which is by John Kander and Fred Ebb.
The character of Sally Bowles was based on Jean Ross, a real–life British cabaret performer (and staunch Marxist) whom Christopher Isherwood met in Berlin in 1929. Ross came from a privileged background; her father was a wealthy Scottish cotton merchant.
In Isherwood's works, Sally Bowles is the daughter of a Lancashire cotton–mill owner. Julie Harris, who played Sally in both the Broadway and film versions of I Am a Camera (1951 and 1955, respectively) was American; but only in the 1972 film version of Cabaret (when she's played by Liza Minnelli) is the character portrayed as American.
© Haydn Thompson 2020