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It's worth noting that Guido van Rossum, creator of Python, is working at Dropbox on this project, and that Mypy is funded primarily by Dropbox (I think). I'm

It's worth noting that Guido van Rossum, creator of Python, is working at Dropbox on this project, and that Mypy is funded primarily by Dropbox (I think). I'm glad that they are doing it, because it's contributed hugely to the Python ecosystem -- especially making it easier for other companies to make the switch to Python 3. I highly recommend mypy for new and current python3 users. It's one of the biggest and best reasons to be using python3 (not that you should need any more :) )

xvector 8y ago
I've never used mypy and I used Python a lot until I recently got my first job. It kind of looks like a band-aid. Could you elaborate on why I would use, say, mypy as opposed to Golang if I don't need the benefits of a scripting language? edit: I can understand why mypy would be useful to refactor an existing (large) project to bring forth type safety guarantees as you go forwards, but surely for new projects there are other languages of choice if type safety is what you want?
- joshuamorton 8y ago
  If python offers type safety, and language x offers type safety, what reason do I have to pick language x over python?
  - rantanplan 8y ago
    5x(probably more) reduction on server costs because of less memory/cpu requirements, if you choose Go for example.
    - joshuamorton 8y ago
      Ah, so your reasons to pick a different language are based not on type safety, but on performance. Indeed there may be a plethora of valid reasons to pick another language over python for some project. But that wasn't the question.
      - rantanplan 8y ago
        Type safety of Go is leaps and bounds better than python's will ever be, with mypy or not.
        joshuamorton 8y ago
        That's just factually untrue. Mypy supports generics, protocols, (via extensions) dependent types, and user-defined covariance rules. Its leaps and bounds more powerful than go's type system. Pytype adds type inference to that list. You end up with a type system that's closer in power to Scala's or Haskell's than Java or Go's.
        rantanplan 8y ago
        Mypy's various bugs are just indicative of how much of a band-aid it is. If you truly believe that a dynamic language like python can have type safety, then imagine what a language like Go can achieve. Python already has deviated too much from where it started and mypy offers too little to justify the verbosity. So I guess we'll see what the future holds for them.
        viraptor 8y ago
        > Mypy's various bugs are just indicative of how much of a band-aid it is. I think you're confusing an idea with an implementation. Bugs can be fixed. If mypy is useful, it will remain useful and slightly more stable.
        joshuamorton 8y ago
        >If you truly believe that a dynamic language like python can have type safety, then imagine what a language like Go can achieve. Literally exactly the same thing, but no more. Mypy and Go's compiler have the exact same information available at compile time. Python stops being a dynamically typed language when you add Mypy. It becomes statically typed. Calling it "dynamic" is like calling C++ "dynamic" because C++ doesn't have access to run-time type information (usually). Actually that's not true, due to the increased expressiveness of Mypy's type system, it has more information available. Mypy has more information. It can do stronger analysis. Go has less information, it can't do as much. Go can make language (syntax) changes to improve this. Python has been very smart, and the type system is implemented as normal objects using the normal syntax. Adding new types, even "special" ones (like Union or Interface) doesn't require modifying the language itself, but can be done only with modifications to a fairly nimble standard library module. This allows python's type system to evolve much more quickly than Go's has shown itself capable of. >Python already has deviated too much from where it started and mypy offers too little to justify the verbosity. But python's type system offers more than Go's, and isn't particularly more verbose, so what are you saying here about Go's type system?
    - tropo 8y ago
      Go has to be better than that. Java beats Python by a factor of 10, and C beats Java by a factor of 10. C is beating Python by a factor of 100. I would expect Go to be a bit faster than Java.
- rantanplan 8y ago
  Because python can't do anything else at this point. It can't be faster. It can't consume less memory. It can't stay still. So they add auxiliary features. No matter how much they downvote you, you're right. For a new project it doesn't make sense to not use Go, or something similar, if you're going down that road.
  - DrJosiah 8y ago
    You're wrong on every point. Congratulations! Cpython (every edition gets optimizations)/PyPy/Numba/Cython/... -> faster. Dict optimizations -> less memory.
- setr 8y ago
  I believe the general idea with gradual typing is that 1) It's faster to write the initial proof of concept with a dynamic language 2) It's common that the proof of concept becomes sufficiently big/stable that it turns into the final codebase 3) Gradual typing makes the path from PoC to real codebase smoother, instead of re-implementing in another language. Point 1 is contentious of course, but that argument isn't relevant here. It is believed, and that belief becomes practice. An example of point 2 might be OSH of the oil shell, which doesn't use mypy I believe, but was originally written in python with the intent to rewrite in C++, and eventually decided it was too costly to rewrite (somehow he decided that ripping apart the python VM would be the more fruitful path... and apparently he's making good progress there). So basically, the usecase of mypy is when you're making the decision of continuing with python, or rewriting. Sometimes rewriting was part of the strategy from the start, other times a script just grew into a full-fledged program over time, but mypy's primary goal wouldn't be to introduce from the start. Another usecase I've personally done is having the typing available to me from the start, but not actually making use of it until I reached a particularly nasty section of code. And so that particular chunk becomes (reasonably) statically typed, while the rest of the program remains standard python. Although in that case I used a library that enforced typechecking at runtime rather than mypy (the library was typeguard iirc), which I only enabled on test runs (it slowed down the program significantly).
  - xiaodai 8y ago
    If you want gradual typing for a smoother ride and performance then just choose Julia. Seriously.
    - happy_man 8y ago
      Let's wait a couple of years until Julia stabilizes itself before suggesting is ready for production
      - xiaodai 8y ago
        Sound advice. But i am sure it's fine to gp head first

xvector 8y ago

I've never used mypy and I used Python a lot until I recently got my first job. It kind of looks like a band-aid. Could you elaborate on why I would use, say, mypy as opposed to Golang if I don't need the benefits of a scripting language? edit: I can understand why mypy would be useful to refactor an existing (large) project to bring forth type safety guarantees as you go forwards, but surely for new projects there are other languages of choice if type safety is what you want?

joshuamorton 8y ago
If python offers type safety, and language x offers type safety, what reason do I have to pick language x over python?
- rantanplan 8y ago
  5x(probably more) reduction on server costs because of less memory/cpu requirements, if you choose Go for example.
  - joshuamorton 8y ago
    Ah, so your reasons to pick a different language are based not on type safety, but on performance. Indeed there may be a plethora of valid reasons to pick another language over python for some project. But that wasn't the question.
    - rantanplan 8y ago
      Type safety of Go is leaps and bounds better than python's will ever be, with mypy or not.
      - joshuamorton 8y ago
        That's just factually untrue. Mypy supports generics, protocols, (via extensions) dependent types, and user-defined covariance rules. Its leaps and bounds more powerful than go's type system. Pytype adds type inference to that list. You end up with a type system that's closer in power to Scala's or Haskell's than Java or Go's.
        rantanplan 8y ago
        Mypy's various bugs are just indicative of how much of a band-aid it is. If you truly believe that a dynamic language like python can have type safety, then imagine what a language like Go can achieve. Python already has deviated too much from where it started and mypy offers too little to justify the verbosity. So I guess we'll see what the future holds for them.
        viraptor 8y ago
        > Mypy's various bugs are just indicative of how much of a band-aid it is. I think you're confusing an idea with an implementation. Bugs can be fixed. If mypy is useful, it will remain useful and slightly more stable.
        joshuamorton 8y ago
        >If you truly believe that a dynamic language like python can have type safety, then imagine what a language like Go can achieve. Literally exactly the same thing, but no more. Mypy and Go's compiler have the exact same information available at compile time. Python stops being a dynamically typed language when you add Mypy. It becomes statically typed. Calling it "dynamic" is like calling C++ "dynamic" because C++ doesn't have access to run-time type information (usually). Actually that's not true, due to the increased expressiveness of Mypy's type system, it has more information available. Mypy has more information. It can do stronger analysis. Go has less information, it can't do as much. Go can make language (syntax) changes to improve this. Python has been very smart, and the type system is implemented as normal objects using the normal syntax. Adding new types, even "special" ones (like Union or Interface) doesn't require modifying the language itself, but can be done only with modifications to a fairly nimble standard library module. This allows python's type system to evolve much more quickly than Go's has shown itself capable of. >Python already has deviated too much from where it started and mypy offers too little to justify the verbosity. But python's type system offers more than Go's, and isn't particularly more verbose, so what are you saying here about Go's type system?
  - tropo 8y ago
    Go has to be better than that. Java beats Python by a factor of 10, and C beats Java by a factor of 10. C is beating Python by a factor of 100. I would expect Go to be a bit faster than Java.
rantanplan 8y ago
Because python can't do anything else at this point. It can't be faster. It can't consume less memory. It can't stay still. So they add auxiliary features. No matter how much they downvote you, you're right. For a new project it doesn't make sense to not use Go, or something similar, if you're going down that road.
- DrJosiah 8y ago
  You're wrong on every point. Congratulations! Cpython (every edition gets optimizations)/PyPy/Numba/Cython/... -> faster. Dict optimizations -> less memory.
setr 8y ago
I believe the general idea with gradual typing is that 1) It's faster to write the initial proof of concept with a dynamic language 2) It's common that the proof of concept becomes sufficiently big/stable that it turns into the final codebase 3) Gradual typing makes the path from PoC to real codebase smoother, instead of re-implementing in another language. Point 1 is contentious of course, but that argument isn't relevant here. It is believed, and that belief becomes practice. An example of point 2 might be OSH of the oil shell, which doesn't use mypy I believe, but was originally written in python with the intent to rewrite in C++, and eventually decided it was too costly to rewrite (somehow he decided that ripping apart the python VM would be the more fruitful path... and apparently he's making good progress there). So basically, the usecase of mypy is when you're making the decision of continuing with python, or rewriting. Sometimes rewriting was part of the strategy from the start, other times a script just grew into a full-fledged program over time, but mypy's primary goal wouldn't be to introduce from the start. Another usecase I've personally done is having the typing available to me from the start, but not actually making use of it until I reached a particularly nasty section of code. And so that particular chunk becomes (reasonably) statically typed, while the rest of the program remains standard python. Although in that case I used a library that enforced typechecking at runtime rather than mypy (the library was typeguard iirc), which I only enabled on test runs (it slowed down the program significantly).
- xiaodai 8y ago
  If you want gradual typing for a smoother ride and performance then just choose Julia. Seriously.
  - happy_man 8y ago
    Let's wait a couple of years until Julia stabilizes itself before suggesting is ready for production
    - xiaodai 8y ago
      Sound advice. But i am sure it's fine to gp head first