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Python docs: rewrite a part

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- fix some mistakes
- move explanation on how to create Python environments to the front
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Frederik Rietdijk 2017-08-26 11:21:30 +02:00
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doc/languages-frameworks/python.md
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@ -2,115 +2,204 @@
## User Guide
Several versions of Python are available on Nix as well as a high amount of
packages. The default interpreter is CPython 2.7.
### Using Python
#### Overview
Several versions of the Python interpreter are available on Nix, as well as a
high amount of packages. The attribute `python` refers to the default
interpreter, which is currently CPython 2.7. It is also possible to refer to
specific versions, e.g. `python35` refers to CPython 3.5, and `pypy` refers to
the default PyPy interpreter.
Python is used a lot, and in different ways. This affects also how it is
packaged. In the case of Python on Nix, an important distinction is made between
whether the package is considered primarily an application, or whether it should
be used as a library, i.e., of primary interest are the modules in
`site-packages` that should be importable.
In the Nixpkgs tree Python applications can be found throughout, depending on
what they do, and are called from the main package set. Python libraries,
however, are in separate sets, with one set per interpreter version.
The interpreters have several common attributes. One of these attributes is
`pkgs`, which is a package set of Python libraries for this specific
interpreter. E.g., the `toolz` package corresponding to the default interpreter
is `python.pkgs.toolz`, and the CPython 3.5 version is `python35.pkgs.toolz`.
The main package set contains aliases to these package sets, e.g.
`pythonPackages` refers to `python.pkgs` and `python35Packages` to
`python35.pkgs`.
#### Installing Python and packages
It is important to make a distinction between Python packages that are
used as libraries, and applications that are written in Python.
The Nix and NixOS manuals explain how packages are generally installed. In the
case of Python and Nix, it is important to make a distinction between whether the
package is considered an application or a library.
Applications on Nix are installed typically into your user
Applications on Nix are typically installed into your user
profile imperatively using `nix-env -i`, and on NixOS declaratively by adding the
package name to `environment.systemPackages` in `/etc/nixos/configuration.nix`.
Dependencies such as libraries are automatically installed and should not be
installed explicitly.
The same goes for Python applications and libraries. Python applications can be
installed in your profile, but Python libraries you would like to use to develop
cannot. If you do install libraries in your profile, then you will end up with
import errors.
installed in your profile. But Python libraries you would like to use for
development cannot be installed, at least not individually, because they won't
be able to find each other resulting in import errors. Instead, it is possible
to create an environment with `python.buildEnv` or `python.withPackages` where
the interpreter and other executables are able to find each other and all of the
modules.
#### Python environments using `nix-shell`
In the following examples we create an environment with Python 3.5, `numpy` and
`toolz`. As you may imagine, there is one limitation here, and that's that
you can install only one environment at a time. You will notice the complaints
about collisions when you try to install a second environment.
The recommended method for creating Python environments for development is with
`nix-shell`. Executing
##### Environment defined in separate `.nix` file
```sh
$ nix-shell -p python35Packages.numpy python35Packages.toolz
Create a file, e.g. `build.nix`, with the following expression
```nix
with import <nixpkgs> {};
python35.withPackages (ps: with ps; [ numpy toolz ])
```
and install it in your profile with
```shell
nix-env -if build.nix
```
Now you can use the Python interpreter, as well as the extra packages (`numpy`,
`toolz`) that you added to the environment.
##### Environment defined in `~/.config/nixpkgs/config.nix`
If you prefer to, you could also add the environment as a package override to the Nixpkgs set, e.g.
using `config.nix`,
```nix
{ # ...
packageOverrides = pkgs: with pkgs; {
myEnv = python35.withPackages (ps: with ps; [ numpy toolz ]);
};
}
```
and install it in your profile with
```shell
nix-env -iA nixpkgs.myEnv
```
The environment is is installed by referring to the attribute, and considering
the `nixpkgs` channel was used.
##### Environment defined in `/etc/nixos/configuration.nix`
For the sake of completeness, here's another example how to install the environment system-wide.
```nix
{ # ...
environment.systemPackages = with pkgs; [
(python35.withPackages(ps: with ps; [ numpy toolz ]))
];
}
```
opens a Nix shell which has available the requested packages and dependencies.
Now you can launch the Python interpreter (which is itself a dependency)
#### Temporary Python environment with `nix-shell`
The examples in the previous section showed how to install a Python environment
into a profile. For development you may need to use multiple environments.
`nix-shell` gives the possibility to temporarily load another environment, akin
to `virtualenv`.
There are two methods for loading a shell with Python packages. The first and recommended method
is to create an environment with `python.buildEnv` or `python.withPackages` and load that. E.g.
```sh
$ nix-shell -p 'python35.withPackages(ps: with ps; [ numpy toolz ])'
```
opens a shell from which you can launch the interpreter
```sh
[nix-shell:~] python3
```
The other method, which is not recommended, does not create an environment and requires you to list the packages directly,
If the packages were not available yet in the Nix store, Nix would download or
build them automatically. A convenient option with `nix-shell` is the `--run`
option, with which you can execute a command in the `nix-shell`. Let's say we
want the above environment and directly run the Python interpreter
```sh
$ nix-shell -p python35.pkgs.numpy python35.pkgs.toolz
```
Again, it is possible to launch the interpreter from the shell.
The Python interpreter has the attribute `pkgs` which contains all Python libraries for that specific interpreter.
##### Load environment from `.nix` expression
As explained in the Nix manual, `nix-shell` can also load an
expression from a `.nix` file. Say we want to have Python 3.5, `numpy`
and `toolz`, like before, in an environment. Consider a `shell.nix` file
with
```nix
with import <nixpkgs> {};
python35.withPackages (ps: [ps.numpy ps.toolz])
```
Executing `nix-shell` gives you again a Nix shell from which you can run Python.
What's happening here?
1. We begin with importing the Nix Packages collections. `import <nixpkgs>` imports the `<nixpkgs>` function, `{}` calls it and the `with` statement brings all attributes of `nixpkgs` in the local scope. These attributes form the main package set.
2. Then we create a Python 3.5 environment with the `withPackages` function.
3. The `withPackages` function expects us to provide a function as an argument that takes the set of all python packages and returns a list of packages to include in the environment. Here, we select the packages `numpy` and `toolz` from the package set.
##### Execute command with `--run`
A convenient option with `nix-shell` is the `--run`
option, with which you can execute a command in the `nix-shell`. We can
e.g. directly open a Python shell
```sh
$ nix-shell -p python35Packages.numpy python35Packages.toolz --run "python3"
```
This way you can use the `--run` option also to directly run a script
or run a script
```sh
$ nix-shell -p python35Packages.numpy python35Packages.toolz --run "python3 myscript.py"
```
In fact, for this specific use case there is a more convenient method. You can
##### `nix-shell` as shebang
In fact, for the second use case, there is a more convenient method. You can
add a [shebang](https://en.wikipedia.org/wiki/Shebang_(Unix)) to your script
specifying which dependencies Nix shell needs. With the following shebang, you
can use `nix-shell myscript.py` and it will make available all dependencies and
specifying which dependencies `nix-shell` needs. With the following shebang, you
can just execute `./myscript.py`, and it will make available all dependencies and
run the script in the `python3` shell.
```py
#! /usr/bin/env nix-shell
#! nix-shell -i python3 -p python3Packages.numpy
#! nix-shell -i 'python3.withPackages(ps: [ps.numpy])'
import numpy
print(numpy.__version__)
```
Likely you do not want to type your dependencies each and every time. What you
can do is write a simple Nix expression which sets up an environment for you,
requiring you only to type `nix-shell`. Say we want to have Python 3.5, `numpy`
and `toolz`, like before, in an environment. With a `shell.nix` file
containing
```nix
with import <nixpkgs> {};
(pkgs.python35.withPackages (ps: [ps.numpy ps.toolz])).env
```
executing `nix-shell` gives you again a Nix shell from which you can run Python.
What's happening here?
1. We begin with importing the Nix Packages collections. `import <nixpkgs>` import the `<nixpkgs>` function, `{}` calls it and the `with` statement brings all attributes of `nixpkgs` in the local scope. Therefore we can now use `pkgs`.
2. Then we create a Python 3.5 environment with the `withPackages` function.
3. The `withPackages` function expects us to provide a function as an argument that takes the set of all python packages and returns a list of packages to include in the environment. Here, we select the packages `numpy` and `toolz` from the package set.
4. And finally, for in interactive use we return the environment by using the `env` attribute.
### Developing with Python
Now that you know how to get a working Python environment with Nix, it is time
to go forward and start actually developing with Python. We will first have a
look at how Python packages are packaged on Nix. Then, we will look at how you
can use development mode with your code.
Now that you know how to get a working Python environment on Nix, it is time to go forward and start actually developing with Python.
We will first have a look at how Python packages are packaged on Nix. Then, we will look how you can use development mode with your code.
#### Packaging a library
#### Python packaging on Nix
On Nix all packages are built by functions. The main function in Nix for building Python packages is [`buildPythonPackage`](https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/interpreters/python/build-python-package.nix).
Let's see how we would build the `toolz` package. According to [`python-packages.nix`](https://raw.githubusercontent.com/NixOS/nixpkgs/master/pkgs/top-level/python-packages.nix) `toolz` is build using
With Nix all packages are built by functions. The main function in Nix for
building Python libraries is `buildPythonPackage`. Let's see how we can build the
`toolz` package.
```nix
{ # ...
toolz = buildPythonPackage rec {
name = "toolz-${version}";
pname = "toolz";
version = "0.7.4";
name = "${pname}-${version}";
src = pkgs.fetchurl {
url = "mirror://pypi/t/toolz/toolz-${version}.tar.gz";
src = fetchPypi {
inherit pname version;
sha256 = "43c2c9e5e7a16b6c88ba3088a9bfc82f7db8e13378be7c78d6c14a5f8ed05afd";
};
doCheck = false;
meta = {
homepage = "http://github.com/pytoolz/toolz/";
description = "List processing tools and functional utilities";
@ -122,63 +211,37 @@ Let's see how we would build the `toolz` package. According to [`python-packages
```
What happens here? The function `buildPythonPackage` is called and as argument
it accepts a set. In this case the set is a recursive set ([`rec`](http://nixos.org/nix/manual/#sec-constructs)).
One of the arguments is the name of the package, which consists of a basename
(generally following the name on PyPi) and a version. Another argument, `src`
specifies the source, which in this case is fetched from an url. `fetchurl` not
only downloads the target file, but also validates its hash. Furthermore, we
specify some (optional) [meta information](http://nixos.org/nixpkgs/manual/#chap-meta).
The output of the function is a derivation, which is an attribute with the name
`toolz` of the set `pythonPackages`. Actually, sets are created for all interpreter versions,
so e.g. `python27Packages`, `python35Packages` and `pypyPackages`.
it accepts a set. In this case the set is a recursive set, `rec`. One of the
arguments is the name of the package, which consists of a basename (generally
following the name on PyPi) and a version. Another argument, `src` specifies the
source, which in this case is fetched from PyPI using the helper function
`fetchPypi`. The argument `doCheck` is used to set whether tests should be run
when building the package. Furthermore, we specify some (optional) meta
information. The output of the function is a derivation.
An expression for `toolz` can be found in the Nixpkgs repository. As explained
in the introduction of this Python section, a derivation of `toolz` is available
for each interpreter version, e.g. `python35.pkgs.toolz` refers to the `toolz`
derivation corresponding to the CPython 3.5 interpreter.
The above example works when you're directly working on
`pkgs/top-level/python-packages.nix` in the Nixpkgs repository. Often though,
you will want to test a Nix expression outside of the Nixpkgs tree. If you
create a `shell.nix` file with the following contents
you will want to test a Nix expression outside of the Nixpkgs tree.
```nix
with import <nixpkgs> {};
pkgs.python35Packages.buildPythonPackage rec {
name = "toolz-${version}";
version = "0.8.0";
src = pkgs.fetchurl {
url = "mirror://pypi/t/toolz/toolz-${version}.tar.gz";
sha256 = "e8451af61face57b7c5d09e71c0d27b8005f001ead56e9fdf470417e5cc6d479";
};
doCheck = false;
meta = {
homepage = "http://github.com/pytoolz/toolz/";
description = "List processing tools and functional utilities";
license = licenses.bsd3;
maintainers = with maintainers; [ fridh ];
};
}
```
and then execute `nix-shell` will result in an environment in which you can use
Python 3.5 and the `toolz` package. As you can see we had to explicitly mention
for which Python version we want to build a package.
The above example considered only a single package. Generally you will want to use multiple packages.
If we create a `shell.nix` file with the following contents
The following expression creates a derivation for the `toolz` package,
and adds it along with a `numpy` package to a Python environment.
```nix
with import <nixpkgs> {};
( let
toolz = pkgs.python35Packages.buildPythonPackage rec {
name = "toolz-${version}";
version = "0.8.0";
my_toolz = python35.pkgs.buildPythonPackage rec {
pname = "toolz";
version = "0.7.4";
name = "${pname}-${version}";
src = pkgs.fetchurl {
url = "mirror://pypi/t/toolz/toolz-${version}.tar.gz";
sha256 = "e8451af61face57b7c5d09e71c0d27b8005f001ead56e9fdf470417e5cc6d479";
src = python35.pkgs.fetchPypi {
inherit pname version;
sha256 = "43c2c9e5e7a16b6c88ba3088a9bfc82f7db8e13378be7c78d6c14a5f8ed05afd";
};
doCheck = false;
@ -189,24 +252,24 @@ with import <nixpkgs> {};
};
};
in pkgs.python35.withPackages (ps: [ps.numpy toolz])
in python35.withPackages (ps: [ps.numpy my_toolz])
).env
```
Executing `nix-shell` will result in an environment in which you can use
Python 3.5 and the `toolz` package. As you can see we had to explicitly mention
for which Python version we want to build a package.
and again execute `nix-shell`, then we get a Python 3.5 environment with our
locally defined package as well as `numpy` which is build according to the
definition in Nixpkgs. What did we do here? Well, we took the Nix expression
that we used earlier to build a Python environment, and said that we wanted to
include our own version of `toolz`. To introduce our own package in the scope of
`withPackages` we used a
[`let`](http://nixos.org/nix/manual/#sec-constructs) expression.
You can see that we used `ps.numpy` to select numpy from the nixpkgs package set (`ps`).
But we do not take `toolz` from the nixpkgs package set this time.
Instead, `toolz` will resolve to our local definition that we introduced with `let`.
So, what did we do here? Well, we took the Nix expression that we used earlier
to build a Python environment, and said that we wanted to include our own
version of `toolz`, named `my_toolz`. To introduce our own package in the scope
of `withPackages` we used a `let` expression. You can see that we used
`ps.numpy` to select numpy from the nixpkgs package set (`ps`). We did not take
`toolz` from the Nixpkgs package set this time, but instead took our own version
that we introduced with the `let` expression.
### Handling dependencies
#### Handling dependencies
Our example, `toolz`, doesn't have any dependencies on other Python
Our example, `toolz`, does not have any dependencies on other Python
packages or system libraries. According to the manual, `buildPythonPackage`
uses the arguments `buildInputs` and `propagatedBuildInputs` to specify dependencies. If something is
exclusively a build-time dependency, then the dependency should be included as a
@ -713,63 +776,6 @@ Both are also exported in `nix-shell`.
## FAQ
### How can I install a working Python environment?
As explained in the user's guide installing individual Python packages
imperatively with `nix-env -i` or declaratively in `environment.systemPackages`
is not supported. However, it is possible to install a Python environment with packages (`python.buildEnv`).
In the following examples we create an environment with Python 3.5, `numpy` and `ipython`.
As you might imagine there is one limitation here, and that's you can install
only one environment at a time. You will notice the complaints about collisions
when you try to install a second environment.
#### Environment defined in separate `.nix` file
Create a file, e.g. `build.nix`, with the following expression
```nix
with import <nixpkgs> {};
pkgs.python35.withPackages (ps: with ps; [ numpy ipython ])
```
and install it in your profile with
```shell
nix-env -if build.nix
```
Now you can use the Python interpreter, as well as the extra packages that you added to the environment.
#### Environment defined in `~/.config/nixpkgs/config.nix`
If you prefer to, you could also add the environment as a package override to the Nixpkgs set.
```nix
{ # ...
packageOverrides = pkgs: with pkgs; {
myEnv = python35.withPackages (ps: with ps; [ numpy ipython ]);
};
}
```
and install it in your profile with
```shell
nix-env -iA nixpkgs.myEnv
```
We're installing using the attribute path and assume the channels is named `nixpkgs`.
Note that I'm using the attribute path here.
#### Environment defined in `/etc/nixos/configuration.nix`
For the sake of completeness, here's another example how to install the environment system-wide.
```nix
{ # ...
environment.systemPackages = with pkgs; [
(python35.withPackages(ps: with ps; [ numpy ipython ]))
];
}
```
### How to solve circular dependencies?
Consider the packages `A` and `B` that depend on each other. When packaging `B`,