diff options
Diffstat (limited to 'doc/user/environments.xml')
| -rw-r--r-- | doc/user/environments.xml | 89 |
1 files changed, 81 insertions, 8 deletions
diff --git a/doc/user/environments.xml b/doc/user/environments.xml index fafb120..5bb1089 100644 --- a/doc/user/environments.xml +++ b/doc/user/environments.xml @@ -672,11 +672,8 @@ environment, of directory names, suffixes, etc. <programlisting> env = Environment() - dict = env.Dictionary() - keys = dict.keys() - keys.sort() - for key in keys: - print "construction variable = '%s', value = '%s'" % (key, dict[key]) + for item in sorted(env.Dictionary().items()): + print "construction variable = '%s', value = '%s'" % item </programlisting> </section> @@ -775,6 +772,83 @@ environment, of directory names, suffixes, etc. </section> <section> + <title>Handling Problems With Value Expansion</title> + + <para> + + If a problem occurs when expanding a construction variable, + by default it is expanded to <literal>''</literal> + (a null string), and will not cause scons to fail. + + <programlisting> + env = Environment() + print "value is:", env.subst( '->$MISSING<-' ) + </programlisting> + + <screen> + % <userinput>scons -Q</userinput> + value is: -><- + scons: `.' is up to date. + </screen> + + This default behaviour can be changed using the &AllowSubstExceptions; + function. + When a problem occurs with a variable expansion it generates + an exception, and the &AllowSubstExceptions; function controls + which of these exceptions are actually fatal and which are + allowed to occur safely. By default, &NameError; and &IndexError; + are the two exceptions that are allowed to occur: so instead of + causing scons to fail, these are caught, the variable expanded to + <literal>''</literal> + and scons execution continues. + To require that all construction variable names exist, and that + indexes out of range are not allowed, call &AllowSubstExceptions; + with no extra arguments. + </para> + + <programlisting> + AllowSubstExceptions() + env = Environment() + print "value is:", env.subst( '->$MISSING<-' ) + </programlisting> + + <screen> + % <userinput>scons -Q</userinput> + value is: + scons: *** NameError `MISSING' trying to evaluate `$MISSING' + File "/home/my/project/SConstruct", line 3, in <module> + </screen> + + <para> + This can also be used to allow other exceptions that might occur, + most usefully with the <literal>${...}</literal> construction + variable syntax. For example, this would allow zero-division to + occur in a variable expansion in addition to the default exceptions + allowed + </para> + + <programlisting> + AllowSubstExceptions(IndexError, NameError, ZeroDivisionError) + env = Environment() + print "value is:", env.subst( '->${1 / 0}<-' ) + </programlisting> + + <screen> + % <userinput>scons -Q</userinput> + value is: -><- + scons: `.' is up to date. + </screen> + <programlisting> + </programlisting> + + <para> + If &AllowSubstExceptions; is called multiple times, each call + completely overwrites the previous list of allowed exceptions. + </para> + + </section> + + <section> <title>Controlling the Default &ConsEnv;: the &DefaultEnvironment; Function</title> <para> @@ -1543,11 +1617,10 @@ environment, of directory names, suffixes, etc. #!/usr/bin/env python import os import sys - if len(sys.argv) > 1: + if len(sys.argv) > 1: keys = sys.argv[1:] else: - keys = os.environ.keys() - keys.sort() + keys = sorted(os.environ.keys()) for key in keys: print " " + key + "=" + os.environ[key] </file> |