We are very experienced travellers and security conscious. We stopped in at new upscale Santa Clara Coffee shop on the San Jacinto Plaza / Saturday art market -- very slick and skilled theive(s) took purse from adjacent chair.
Be very careful and never let your gurard down; keep everything teathered, even when in a seemingly respectable place. Now I know why the Starbucks in Condessa had *2* ! security guards - the hypervigilance required in Mexcico City detracts from the overall experience.
I hope the merchants in San Jacinto Plaza (and the Police, who did not seem overly interested in the theft) do something to deal with the crime in their market, otherwise I would not recommend shopping there.