I think a key thing here is your identification of a paragraph. Nobody would think to exert copyright over individual words. Phrases and epigrams are considered worthy of attribution, but only in exceptional cases. Copying sentences is starting to get into plagiarism, though single sentences would usually be forgiven because noting or remembering a single sentence while forgetting the source is an easy mistake to make. Copying whole paragraph, by contrast, is unlikely to be casual.

I think in programming therms a useful parallel might be copying at the module rather than the statement or function level. For example, if I write some code prompts to do the following:

  - validate my API key with Twitter
  - solicit the input of a Twitter username
  - download the up to 500 of that user's tweets
  - convert the json to a dataframe
  - plot the derivative of the intervals between tweets

...many of those tasks can be fairly described as helper functions, either taken directly from documentation (like interfacing with an API) or being so elementary as to be generic. If any one of these tasks happened to come from your code or mine, and the rest from other programs, it wouldn't feel like much of an infringement. If all of them came from the same body of code, it would.