python/colour: add colourizing and switch functions

When samba.colour is first imported, the function

colour.c_BLUE("samba")

will give you the string "\033[1;34msamba\033[0m", which will show up
as blue on an ANSI terminal. If you then go:

colour.switch_colour_off()
colour.c_BLUE("samba")

the c_BLUE call will return the uncoloured string "samba".

This is so things like samba-tool can do this sort of thing:

    if not os.isatty(self.outf):
        switch_colour_off()

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

python/samba/colour.py

@@ -18,15 +18,13 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #
 # The 4 bit colours are available as global variables with names like
-# RED, DARK_RED, REV_RED (for red background), and REV_DARK_RED.
+# RED, DARK_RED, REV_RED (for red background), and REV_DARK_RED. If
+# switch_colour_off() is called, these names will all point to the
+# empty string. switch_colour_on() restores the default values.
 #
 # The 256-colour codes are obtained using xterm_256_color(n), where n
 # is the number of the desired colour.
 
-# C_NORMAL resets to normal, whatever that is
-C_NORMAL = "\033[0m"
-
-UNDERLINE = "\033[4m"
 
 def _gen_ansi_colours():
     g = globals()
@@ -37,11 +35,53 @@ def _gen_ansi_colours():
         g['REV_' + name] = "\033[1;4%dm" % i
         g['REV_DARK_' + name] = "\033[4%dm" % i
 
+    # kcc.debug uses these aliases (which make visual sense)
+    g['PURPLE'] = DARK_MAGENTA
+    g['GREY'] = DARK_WHITE
+
+    # C_NORMAL resets to normal, whatever that is
+    g['C_NORMAL'] = "\033[0m"
+
+    # Non-colour ANSI codes.
+    g['UNDERLINE'] = "\033[4m"
+
+
 _gen_ansi_colours()
 
-# kcc.debug uses these aliases (which make visual sense)
-PURPLE = DARK_MAGENTA
-GREY = DARK_WHITE
+# Generate functions that colour a string. The functions look like
+# this:
+#
+#    c_BLUE("hello")  # "\033[1;34mhello\033[0m" -> blue text
+#    c_DARK_RED(3)    # 3 will be stringified and coloured
+#
+# but if colour is switched off, no colour codes are added.
+#
+#    c_BLUE("hello")  # "hello"
+#
+# The definition of the functions looks a little odd, because we want
+# to bake in the name of the colour but not its actual value.
+
+for _k in list(globals().keys()):
+    if _k.isupper():
+        def _f(s, name=_k):
+            return "%s%s%s" % (globals()[name], s, C_NORMAL)
+        globals()['c_%s' % _k] = _f
+
+del _k, _f
+
+
+def switch_colour_off():
+    """Convert all the ANSI colour codes into empty strings."""
+    g = globals()
+    for k, v in list(g.items()):
+        if k.isupper() and isinstance(v, str) and v.startswith('\033'):
+            g[k] = ''
+
+
+def switch_colour_on():
+    """Regenerate all the ANSI colour codes."""
+    _gen_ansi_colours()
+
 
 def xterm_256_colour(n, bg=False, bold=False):
     weight = '01;' if bold else ''