Conditional settings, options and requirements

Remember, in your conanfile.py you have also access to the options of your dependencies, and you can use them to:

Add requirements dynamically
Change values of options

The configure method might be used to hardcode dependencies options values. It is strongly discouraged to use it to change the settings values, please remember that settings are a configuration input, so it doesn’t make sense to modify it in the recipes.

Also, for options, a more flexible solution is to define dependencies options values in the default_options, not in the configure() method, as this would allow to override them. Hardcoding them in the configure() method won’t allow that and thus won’t easily allow conflict resolution. Use it only when it is absolutely necessary that the package dependencies use those options.

Here is an example of what we could do in our configure method:

...
requires = "Poco/1.9.0@pocoproject/stable" # We will add OpenSSL dynamically "OpenSSL/1.0.2d@lasote/stable"
...

def configure(self):
    # We can control the options of our dependencies based on current options
    self.options["OpenSSL"].shared = self.options.shared

    # Maybe in windows we know that OpenSSL works better as shared (false)
    if self.settings.os == "Windows":
       self.options["OpenSSL"].shared = True

       # Or adjust any other available option
       self.options["Poco"].other_option = "foo"

    # We could check the presence of an option
    if "shared" in self.options:
        pass

def requirements(self):
    # Or add a new requirement!
    if self.options.testing:
       self.requires("OpenSSL/2.1@memsharded/testing")
    else:
       self.requires("OpenSSL/1.0.2d@lasote/stable")

Constrain settings and options

Sometimes there are libraries that are not compatible with specific settings like libraries that are not compatible with an architecture or options that only make sense for an operating system. It can be also useful when there are settings under development.

There are two approaches for this situation:

Use configure() to raise an error for non-supported configurations:

This approach is the first one evaluated when Conan loads the recipe so it is quite handy to perform checks of the input settings. It relies on the set of possible settings inside your settings.yml file so it can be used to constrain any recipe.
```
def configure(self):
    if self.settings.os == "Windows":
      raise ConanException("This library is not compatible with Windows")
```
This same method is also valid for options and config_options() method and it is commonly used to remove options for one setting:
```
def config_options(self):
    if self.settings.os == "Windows":
        del self.options.fPIC
```
Constrain settings inside a recipe:

This approach constrains the settings inside a recipe to a subset of them and it is normally used in recipes that are never supposed to work out of the restricted settings.
```
from conans import ConanFile

class MyConan(ConanFile):
    name = "myconanlibrary"
    version = "1.0.0"
    settings = {"os": None, "build_type": None, "compiler": None, "arch": ["x86_64"]}
```
The disadvantage of this is that possible settings are hardcoded in the recipe and in case new values are used in the future, it will require the recipe to be modified explicitly.

Important

Note the use of None value in the os, compiler and build_type settings described above will allow them to take the values from settings.yml file

We strongly recommend the use if the first approach whenever it is possible and use the second one only for those cases where a stronger constrain is needed for a particular recipe.