a no ser por =~ unless; with the exception of
people often get it mixed up with "de no ser por"
Some examples to grasp the difference
De no ser por él, el barco se hundía. [He was the exceptional factor --> "de..", sort of "it comes from him"]
A no ser por él, todos estuvieron de acuerdo. [He was the exception to the rule ---> "a..." sort of "it ends at him"]
Both phrases "it comes from him" and "it ends at him" are not truly the meaning but they may trigger reflexion about "de no ser por" and "a no ser por" both managing exceptions but in a fairly different way.
This is how people get those mixed up:
De no ser porque la pintura no se ha secado aún, ya podríamos dormir en esta casa.
Ya está todo listo, a no ser por la pintura que todavía no se secó.
Because the situation is the same (some element lacking in order to complete a whole) people invent some dubious phrases:
A no ser por la pintura que no se secó, ya dormíamos aquí.
Which is a "brainsore" when an educated person tries to parse it, as it also contains an unconventional use of tenses.