Given that the police did not bother to investigate there is no wonder the DA did not "press charges". I do not really get the grand parent post. Police investigation happens before any charges are pressed. So no matter how incompetent the DA is I do not see how they could possibly be to blame here.

People have trouble disentangling the problematic behavior of the police and the consequences of prosecutorial discretion.

The OP's argument is likely that the police didn't arrest an attacker because they knew that the DA wouldn't do anything. There is the plausible alternate explanation that the police were engaged in a bad-faith work stoppage or slow down. The third explanation--nonunique to DA--is that the police were overworked or incompetent.

The third explanation is the only one that makes sense. SF police haven't done their jobs before or after Boudin either. The Tenderloin has had open crime for decades, and that didn't change just because there was a new DA.

They are not mutually exclusive either. It could have been all three. Let me also add that morale is an issue. The police have been taught to value their work in a certain kind of way (chase the bad guy, lock him up, get rewarded), but the world is changing. A lot of these cops feel betrayed by new political positions like "don't arrest for theft," and they legitimately start to wonder what is the point of doing a difficult and dangerous job if they don't have support in their own (I cringe) chain of command? For an external observer it seems like this should not prevent police from e.g. arresting people for murder, but it really has an impact on outcomes.

The police and the DA both have the police reports and all the info I discussed. The DA has plenty of information on the situation to bring charges.

Because if the DA says he won't prosecute certain crimes, why would the police bother to arrest someone that will be released on no cash bail. Why would they risk themselves or being in a situation where they need to use force and come under public scrutiny?

In many of these cities, these DAs are propped up with insane campeign funds, get elected, and refuse to charge, or push for no bail, so someone gets arrested for a felony, they end up back on the street the next day. Just read some of the cases where some people are arrested over and over and keep reoffending before their trial date.

The "certain crimes" that the DA said he would not prosecute are, in his words, "Crimes such as public camping, offering or soliciting sex, public urination, blocking a sidewalk, etc.".

It's a bit of a leap from there to conclude that attempted murder would not be prosecuted.

It's a different DA, but the current Manhattan DA (who appears to be from the same school of thought as the one under discussion), upon taking office in January 2022, issued a written policy memorandum which stated that his office will stop prosecuting, among other things, trespass, jumping the turnstiles, and driving without a license or on a suspended license; and further downgraded burglaries and drug cases, in some cases from what the text of the law classifies as a felony down to a misdemeanor.

Does it say anywhere on that memo that attempted murder would not be prosecuted? Of course not. Do these policies generally increase the feeling of lawlessness in the jurisdiction? Yes. Might that precipitate higher degrees of lawlessness? You be the judge.

You're making a "broken windows" argument here, which may or may not be true. But what I was discussing was a different question: Whether the DAs in question classified stabbing / attempted murder among those crimes not to be prosecuted. And I have not seen any evidence that they have.