How to Fix Urllib3 Runtimeerror: Requests Dependency 'Urllib3' Must Be Version >= 1.21.1, < 1.22

Python (pip) - RequestsDependencyWarning: urllib3 (1.9.1) or chardet (2.3.0) doesn't match a supported version

You do have a mixed setup (both apt and pip were used to install system-wide, which is common), and it indeed doesn't match the supported versions of modules required by requests (and pip v1.5.6 is also quite old).

The requests (which version? likely leftover from pip install) requires:

urllib3: 1.21.1 - 1.22

chardet: 3.0.2 - 3.1.0

You have:

urllib3 (1.9.1) from python-urllib3 1.9.1-3 debian package

chardet (2.3.0) from python-chardet 2.3.0-1 debian package

Two options:

• either downgrade requests to the version from your OS distribution (see what's available with apt show python-requests), or older versions at pypi.org, or

• or install newer urllib3 and chardet (you can download the wheel files manually from pipy.org and do pip install on them, including any dependencies), either at user level (--user pip install option) or in a virtualenv.

You can test everything in a virtualenv (apt show python-virtualenv). It should even deploy a newer pip for you inside of its virtual envs. It is also possible to install a newer pip 10.0.1 at the user-level (--user) alongside your OS-vendored pip but you need to be careful about that. Good luck!

Subclasses and return types

Your first solution is pretty good, IMHO. But let's talk about the questions. First, about what is wrong here:

abstract class PDF[T] {
  def fit[S <: PDF[T]](obs: Seq[T], weights: Seq[Double]): S
}

class PDFGaussian(val mu: Double, val Sigma: Double) extends PDF[Double] {
  def fit[S <: PDF[_]](obs: Seq[Double], weights: Seq[Double]): S =
    new PDFGaussian(...) // bla bla bla
}

Let's say I have

class FooBar extends PDF[Double] { ... }

And I do:

val pdfg = new PDFGaussian(1.0, -1.0)
val foobar = pdfg.fit[FooBar](List(0.5, 0.75), List(4, 2))

So, I'm telling the compiler that I want S to be FooBar, but you are returning PDFGaussian! That's what the compiler is complaining about.

So, how to solve it? Well... tough. :-) How about this:

abstract class PDF[T] {
  type S <: PDF[T]
  def fit(obs: Seq[T], weights: Seq[Double]): S
}

class PDFGaussian(val mu: Double, val Sigma: Double) extends PDF[Double] {
  type S = PDFGaussian
  def fit(obs: Seq[Double], weights: Seq[Double]): S =
    new PDFGaussian(...) // bla bla bla
}

It's a bit more verbose, but it keeps PDF type signature cleaner.

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