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Python requires: reusing code [EXPERIMENTAL]

Warning

This is an experimental feature subject to breaking changes in future releases.

The python_requires() feature is a very convenient way to share files and code between different recipes. A Python Requires is just like any other recipe, it is the way it is required from the consumer what makes the difference.

The Python Requires recipe file, besides exporting its own required sources, can export files to be used by the consumer recipes and also python code in the recipe file itself.

Let's have a look at an example showing all its capabilities (you can find all the sources in Conan examples repository):

  • Python requires recipe:

    import os
    import shutil
    from conans import ConanFile, CMake, tools
    from scm_utils import get_version
    
    
    class PythonRequires(ConanFile):
        name = "pyreq"
        version = "version"
    
        exports = "scm_utils.py"
        exports_sources = "CMakeLists.txt"
    
    
    def get_conanfile():
    
        class BaseConanFile(ConanFile):
    
            settings = "os", "compiler", "build_type", "arch"
            options = {"shared": [True, False]}
            default_options = {"shared": False}
            generators = "cmake"
            exports_sources = "src/*"
    
            def source(self):
                # Copy the CMakeLists.txt file exported with the python requires
                pyreq = self.python_requires["pyreq"]
                shutil.copy(src=os.path.join(pyreq.exports_sources_folder, "CMakeLists.txt"),
                            dst=self.source_folder)
    
                # Rename the project to match the consumer name
                tools.replace_in_file(os.path.join(self.source_folder, "CMakeLists.txt"),
                                      "add_library(mylibrary ${sources})",
                                      "add_library({} ${{sources}})".format(self.name))
    
    
            def build(self):
                cmake = CMake(self)
                cmake.configure()
                cmake.build()
    
            def package(self):
                self.copy("*.h", dst="include", src="src")
                self.copy("*.lib", dst="lib", keep_path=False)
                self.copy("*.dll", dst="bin", keep_path=False)
                self.copy("*.dylib*", dst="lib", keep_path=False)
                self.copy("*.so", dst="lib", keep_path=False)
                self.copy("*.a", dst="lib", keep_path=False)
    
            def package_info(self):
                self.cpp_info.libs = [self.name]
    
        return BaseConanFile
  • Consumer recipe

    from conans import ConanFile, python_requires
    
    
    base = python_requires("pyreq/version@user/channel")
    
    class ConsumerConan(base.get_conanfile()):
        name = "consumer"
        version = base.get_version()
    
        # Everything else is inherited

We must make available for other to use the recipe with the Python Requires, this recipe won't have any associated binaries, only the sources will be needed, so we only need to execute the export and upload commands:

$ conan export . pyreq/version@user/channel
$ conan upload pyreq/version@user/channel -r=myremote

Now any consumer will be able to reuse the business logic and files available in the recipe, let's have a look at the most common use cases.

Import a python requires

To import a recipe as a Python requires it is needed to call the python_requires() function with the reference as the only parameter:

base = python_requires("pyreq/version@user/channel")

All the code available in the conanfile.py file of the imported recipe will be available in the consumer through the base variable.

Important

There are several important considerations regarding python_requires():

  • They are required at every step of the conan commands. If you are creating a package that python_requires("MyBase/..."), the MyBase package should be already available in the local cache or to be downloaded from the remotes. Otherwise, conan will raise a "missing package" error.
  • They do not affect the package binary ID (hash). Depending on different version, or different channel of such python_requires() do not change the package IDs as the normal dependencies do.
  • They are imported only once. The python code that is reused is imported only once, the first time it is required. Subsequent requirements of that conan recipe will reuse the previously imported module. Global initialization at parsing time and global state are discouraged.
  • They are transitive. One recipe using python_requires() can be also consumed with a python_requires() from another package recipe.
  • They are not automatically updated with the --update argument from remotes.
  • Different packages can require different versions in their python_requires(). They are private to each recipe, so they do not conflict with each other, but it is the responsibility of the user to keep consistency.
  • They are not overridden from downstream consumers. Again, as they are private, they are not affected by other packages, even consumers

Reuse python sources

In the example proposed we are using two functions through the base variable: base.get_conanfile() and base.get_version(). The first one is defined directly in the conanfile.py file, but the second one is in a different source file that was exported together with the pyreq/version@user/channel recipe using the exports attribute.

This works without any Conan magic, it is just plain Python and you can even return a class from a function and inherit from it. That's just what we are proposing in this example: all the business logic in contained in the Python Requires so every recipe will reuse it automatically. The consumer only needs to define the name and version:

from conans import ConanFile, python_requires


base = python_requires("pyreq/version@user/channel")

class ConsumerConan(base.get_conanfile()):
    name = "consumer"
    version = "version"

    # Everything else is inherited

while all the functional code is defined in the python requires recipe file:

from conans import ConanFile, python_requires

[...]

def get_conanfile():

    class BaseConanFile(ConanFile):
        def source(self):
            [...]

        def build(self):
            [...]

Reuse source files

Up to now, we have been reusing python code, but we can also package files within the python requires recipe and consume them afterward, that's what we are doing with a CMakeList.txt file, it will allow us to share the CMake code and ensure that all the libraries using the same python requires will have the same build script. These are the relevant code snippets from the example files:

  • The python requires exports the needed sources (the file exists next to this conanfile.py):

    class PythonRequires(ConanFile):
        name = "pyreq"
        version = "version"
    
        exports_sources = "CMakeLists.txt"
    
        [...]

    The file will be exported together with the recipe pyreq/version@user/channel during the call to conan export . pyreq/version@user/channel as it is expected for any Conan package.

  • The consumer recipe will copy the file from the python requires folder, we need to make this copy ourselves, there is nothing run automatically during the python_requires() call:

    class BaseConanFile(ConanFile):
        [...]
    
        def source(self):
            # Copy the CMakeLists.txt file exported with the python requires
            pyreq = self.python_requires["pyreq"]
            shutil.copy(src=os.path.join(pyreq.exports_sources_folder, "CMakeLists.txt"),
                        dst=self.source_folder)
    
            # Rename the project to match the consumer name
            tools.replace_in_file(os.path.join(self.source_folder, "CMakeLists.txt"),
                                  "add_library(mylibrary ${sources})",
                                  "add_library({} ${{sources}})".format(self.name))

    As you can see, in the inherited source() method, we are copying the CMakeLists.txt file from the exports_sources folder of the python requires (take a look at the :ref:`python_requires attribute<python_requires_attribute>`), and modifying a line to name the library with the current recipe name.

    In the example, our ConsumerConan class will also inherit the build(), package() and package_info() method, turning the actual conanfile.py of the library into a mere declaration of the name and version.

You can find the full example in the Conan examples repository.

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