de pronto = de repente
"De repente" may be an itsy bitty more colloquial because there is an exact proper formal adverb with the same origin:
repentinamente
"De pronto" isn't formal or informal.
Quote:
Originally Posted by chileno
Por de pronto = for the moment but you cannot say por de repente.... 
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Por LO pronto ... (LO pronto = the nearest future)
Por lo pronto ... = sort of {"for the time being" but "short time" / "until it changes" / "a new element is needed for it to change"} with touches of "as far as we know".