A user may define a Scanner object to scan a type of file for implicit dependencies:

	def scanner1(file_contents):
	        # search for dependencies
	        return dependency_list

	FirstScan = Scanner(function = scanner1)
	

The scanner function must return a list of dependencies that its finds based on analyzing the file contents it is passed as an argument.

The scanner function, when invoked, will be passed the calling environment. The scanner function can use construction environments from the passed environment to affect how it performs its dependency scan--the canonical example being to use some sort of search-path construction variable to look for dependency files in other directories:

	def scanner2(file_contents, env):
	        path = env.{'SCANNERPATH'}	# XXX
	        # search for dependencies using 'path'
	        return dependency_list

	SecondScan = Scanner(function = scanner2)
	

The user may specify an additional argument when the Scanner object is created. When the scanner is invoked, the additional argument will be passed to the scanner funciton, which can be used in any way the scanner function sees fit:

	def scanner3(file_contents, env, arg):
	        # skip 'arg' lines, then search for dependencies
	        return dependency_list

	Skip_3_Lines_Scan = Scanner(function = scanner2, argument = 3)
	Skip_6_Lines_Scan = Scanner(function = scanner2, argument = 6)
	

A method exists to return a copy of an existing Scanner object, with any overridden values specified as keyword arguments to the method:

	scan = Scanner(function = my_scan)
	scan_path = scan.Copy(path = '%SCANNERPATH')
	

Typically, Scanner objects will be supplied by a tool-master or administrator through a shared construction environment.

Each construction environment has a SCANNERMAP, a dictionary that associates different file suffixes with a scanner object that can be used to generate a list of dependencies from the contents of that file. This SCANNERMAP can be initialized at instantiation:

	env = Environment(SCANNERMAP = {
	                        '.c' : CScan,
	                        '.cc' : CScan,
	                        '.m4' : M4Scan,
	                })
	

Scanner objects referenced in the SCANNERMAP do not need to be listed separately in the SCANNERS variable. The construction environment will bind the union of the Scanner objects listed in both variables.