Improve information about signature directory

Rejects #8.

README.md

@@ -16,7 +16,8 @@ The recommended way is:
 * Use an application of your choice to sign it.
   You can for example use Okular's Freehand Line, or transfer it to your smartphone and use Adobe Acrobat Reader.
   Keep in mind that it's the center of this mini-page that will be used for positioning the signature.
-* Put the signed file in `~/.pdf_signatures/`.
+* Put the signed file in your signature directory.
+  The signature directory is `$PDF_SIGNATURE_DIR`, `$XDG_CONFIG_HOME/pdf_signatures`, `$HOME/.config/pdf_signatures` or `$HOME/.pdf_signatures/`; the first one that exists. Use `pdf-sign -h` to confirm which one will be used on your system.
 
 You can now use the `pdf-sign` tool interactively (or non-interactively) to sign PDF files.
 The GUI is self documented and allows both keyboard-only and pointer-only operation.

pdf-sign

@@ -397,20 +397,44 @@ def qpdfOrPdftk(qpdfCmd, pdftkCmd):
     return True # Some lambdas above rely on this
 
 def getSignatureDir():
-    if 'PDF_SIGNATURE_DIR' in os.environ:
-        sd=os.environ['PDF_SIGNATURE_DIR']
-    elif 'XDG_CONFIG_HOME' in os.environ:
-        sd=os.path.join(os.environ['XDG_CONFIG_HOME'], 'pdf_signatures')
-    elif os.path.exists(os.path.expanduser("~/.config/pdf_signatures")):
-        sd="~/.config/pdf_signatures"
-    else:
-        sd="~/.pdf_signatures"
-    sd=os.path.expanduser(sd)
-    if not os.path.exists(sd):
-        raise Exception(f'Signature directory {sd} does not exist')
-    if not os.path.isdir(sd):
-        raise Exception(f'Signature directory {sd} is not a directory')
-    return sd
+    (path, helptxt) = getSignatureDirAndHelp()
+    if not path:
+        die(f"Could not find a valid signature directory. The options considered are: {helptxt}")
+    return path
+
+def getSignatureDirAndHelp():
+    def candidate(prevpath, prevhelptxt, nr, lbl, envvar, path):
+        helptxt = f"{nr}) {lbl}"
+        if prevhelptxt: helptxt = f"{prevhelptxt} {helptxt}"
+        if envvar and not envvar in os.environ:
+            helptxt += " which is not used since the environment variable is not set."
+            return (prevpath, helptxt)
+        path = os.path.expanduser(path)
+        if not os.path.exists(path):
+            helptxt += f" which is not used since {path} does not exist."
+            return (prevpath, helptxt)
+        if not os.path.isdir(path):
+            helptxt += f" which is not used since {path} is not a directory."
+            return (prevpath, helptxt)
+        if prevpath:
+            helptxt += f" which is valid, but not used since it's not the highest priority directory."
+            return (prevpath, helptxt)
+        helptxt += f" which resolves to {path} and IS USED as the signature directory."
+        return (path, helptxt)
+    (path, helptxt) = (None, None)
+    (path, helptxt) = candidate(path, helptxt,
+        1, '$PDF_SIGNATURE_DIR', 'PDF_SIGNATURE_DIR',
+        os.environ.get('PDF_SIGNATURE_DIR'))
+    (path, helptxt) = candidate(path, helptxt,
+        2, '$XDG_CONFIG_HOME/pdf_signatures', 'XDG_CONFIG_HOME',
+        os.path.join(os.environ.get('XDG_CONFIG_HOME') or '/', 'pdf_signatures'))
+    (path, helptxt) = candidate(path, helptxt,
+        3, '~/.config/pdf_signatures', None,
+        '~/.config/pdf_signatures')
+    (path, helptxt) = candidate(path, helptxt,
+        4, '~/.pdf_signatures', None,
+        '~/.pdf_signatures')
+    return (path, helptxt)
 
 # Simple dependency tracking.
 # Init with a value or function to calculate the value.
@@ -536,10 +560,24 @@ if not 'BooleanOptionalAction' in dir(argparse):
             return ' | '.join(self.option_strings)
     argparse.BooleanOptionalAction = BooleanOptionalAction
 
-parser = argparse.ArgumentParser(description='Sign a PDF file.')
+
+def getSignatureHelp():
+    (path, helptxt) = getSignatureDirAndHelp()
+    ret="""
+        Path to file used as signature.
+        Required in batch mode.
+        In GUI mode, the user can choose among PDF files in the signature directory.
+    """
+    if path:
+        ret += f"The signature directory is {path}, chosen from these options: {helptxt}"
+    else:
+        ret += f"Right now, no signature directory could be found. The options are: {helptxt}"
+    return ret
+
+parser = argparse.ArgumentParser(description='Sign a PDF file, with a non-cryptographic squiggle.')
 parser.add_argument('input', metavar='input.pdf', type=str, help='Input PDF file.')
 parser.add_argument('-p', '--page', type=int, default=-1, help='The page to sign, negative for counting from the end. (default: -1)')
-parser.add_argument('-s', '--signature', type=str, help='Path to file used as signature. Required in batch mode. In GUI mode, the user can choose among files in $PDF_SIGNATURE_DIR, $XDG_CONFIG_HOME/pdf_signatures (defaulting to ~/.config/pdf_signatures), or ~/.pdf_signatures.')
+parser.add_argument('-s', '--signature', type=str, help=getSignatureHelp())
 parser.add_argument('-x', '--x-coordinate', type=float, default=0.5, help='Horizontal coordinate of signature center, in page width units. (default: 0.5)')
 parser.add_argument('-y', '--y-coordinate', type=float, default=0.75, help='Vertical coordinate of signature center, in page height units. (default: 0.75)')
 parser.add_argument('-W', '--width', type=float, default=0.28, help='Width of box to fit signature to, in page width units. (default: 0.28)')