iPad Mini fits (barely) in my pants pocket when I'm wearing my baggy cargo pants. Also would go around nicely in a bag. There's a variety of keyboard cases for it. Including hinged ones with trackpads which turn it into a tiny laptop. Can get it with a cellular connection. Small enough to take with you everywhere but big enough to make it awkward to pull out constantly.

Only drawback for your use case is the programming part. Though you could also get a small Samsung or Lenovo 8" Android tablet which would give you much more flexibility. There's also the 8" laptops from a few niche companies, though they will probably be thicker than tablet + keyboard case and wouldn't have the same battery life or cellular options.

Smallest, easily available device I can think of in this form factor is a 7" Amazon Fire with the Zagg hinged keyboard case. It's slow, has so-so battery life, and you'd still need to get a cellular connection elsewhere. But it's about the size of a VHS tape and it's pocketable. I had one for a while because I wanted a cheap tiny laptop. Fire tablets are easy to turn into mostly generic Android machines so it would be useful.