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Hi. The only place where you set the value of INSTRUCAO_GBAK is the init function. There, you have no path to the database yet, i.e., both LOCAL_BASE and LOCAL_BACKUP are empty strings. Before you call the subprocess, see what you've actually pass as the arguments.
To fix the mistake, (option 1) make INSTRUCAO_GBAK in seleciona_bd and salvar_backup functions thee same way you do in the init or (option 2) make INSTRUCAO_GBAK a property to calculate it each time it's used.
Anyway, it's not a PySide topic.