An environment is a collection of values that can affect how a program executes. SCons distinguishes between three different types of environments that can affect the behavior of SCons itself (subject to the configuration in the SConscript files), as well as the compilers and other tools it executes:
The external environment is the set of variables in the user's environment at the time the user runs SCons. These variables are available within the SConscript files through the Python os.environ dictionary. See Section 7.1, below.
A construction environment is a distinct object creating within a SConscript file and and which contains values that affect how SCons decides what action to use to build a target, and even to define which targets should be built from which sources. One of the most powerful features of SCons is the ability to create multiple construction environments, including the ability to clone a new, customized construction environment from an existing construction environment. See Section 7.2, below.
An execution environment is the values that SCons sets when executing an external command (such as a compiler or linker) to build one or more targets. Note that this is not the same as the external environment (see above). See Section 7.3, below.
Unlike Make, SCons does not automatically copy or import values between different environments (with the exception of explicit clones of construction environments, which inherit values from their parent). This is a deliberate design choice to make sure that builds are, by default, repeatable regardless of the values in the user's external environment. This avoids a whole class of problems with builds where a developer's local build works because a custom variable setting causes a different comiler or build option to be used, but the checked-in change breaks the official build because it uses different environment variable settings.
Note that the SConscript writer can easily arrange for variables to be copied or imported between environments, and this is often very useful (or even downright necessary) to make it easy for developers to customize the build in appropriate ways. The point is not that copying variables between different environments is evil and must always be avoided. Instead, it should be up to the implementer of the build system to make conscious choices about how and when to import a variable from one environment to another, making informed decisions about striking the right balance between making the build repeatable on the one hand and convenient to use on the other.
The external environment
variable settings that
the user has in force
when executing SCons
are available through the normal Python
This means that you must add an
import os statuement
to any SConscript file
in which you want to use
values from the user's external environment.
More usefully, you can use the
dictionary in your SConscript
files to initialize construction environments
with values from the user's external environment.
See the next section,
for information on how to do this.