Accept only the supported paper sizes in --paper

README.org

@@ -513,7 +513,7 @@ slip39 --help 2>&1                | sed 's/^/: /' # (just for output formatting)
     :                         quarter, photo or '(<h>,<w>),<margin>' (default:
     :                         business)
     :   --no-card             Disable PDF SLIP-39 mnemonic card output
-    :   --paper PAPER         Paper size (default: Letter)
+    :   --paper PAPER         Paper size; Letter, Legal, A4, Photo (default: Letter)
     :   --cover               Produce PDF SLIP-39 cover page
     :   --no-cover            Disable PDF SLIP-39 cover page
     :   --identifier IDENTIFIER

slip39/main.py

@@ -28,7 +28,7 @@
 from .util		import log_cfg, log_level, input_secure
 from .layout		import write_pdfs
 from .defaults		import (   # noqa: F401
-    ANONYMOUS, CARD, CARD_SIZES, PAPER, WALLET, WALLET_SIZES,
+    ANONYMOUS, CARD, CARD_SIZES, PAPER, PAPER_FORMATS, WALLET, WALLET_SIZES,
     BITS, BITS_DEFAULT,
     FILENAME_FORMAT,
     FILENAME_KEYWORDS,
@@ -43,6 +43,18 @@
 log				= logging.getLogger( __package__ )
 
 
+def paper_format( name ):
+    """Convert a (case-insensitive) paper name into its PAPER_FORMATS value (a paper format name, or
+    (width, height) in mm).  Other sizes aren't supported: the cards and Paper Wallets must fit on the
+    page, and eg. A5 is too small for a Paper Wallet.
+
+    """
+    for paper,fmt in PAPER_FORMATS.items():
+        if paper.lower() == name.lower():
+            return fmt
+    raise argparse.ArgumentTypeError( f"Unsupported paper size {name!r}; use one of: {', '.join( PAPER_FORMATS )}" )
+
+
 def main( argv=None ):
     ap				= argparse.ArgumentParser(
         description = "Create and output SLIP-39 encoded Seeds and Paper Wallets to a PDF file.",
@@ -115,8 +127,8 @@ def main( argv=None ):
     ap.add_argument( '--no-card', dest="card", action='store_false',
                      help="Disable PDF SLIP-39 mnemonic card output" )
     ap.add_argument( '--paper',
-                     default=None,
-                     help=f"Paper size (default: {PAPER})" )
+                     default=None, type=paper_format,
+                     help=f"Paper size; {', '.join( PAPER_FORMATS )} (default: {PAPER})" )
     ap.add_argument( '--cover', dest="cover_page", action='store_true',
                      default=True,
                      help="Produce PDF SLIP-39 cover page" )

slip39/main_test.py

@@ -0,0 +1,31 @@
+import argparse
+import pytest
+
+from .defaults		import PAPER_FORMATS
+from .main		import main, paper_format
+
+
+def test_paper_format():
+    """Paper sizes are case-insensitive names from PAPER_FORMATS, converted to their format value."""
+    assert paper_format( 'a4' ) == paper_format( 'A4' ) == PAPER_FORMATS['A4']
+    assert paper_format( 'photo' ) == PAPER_FORMATS['Photo']  # eg. (w,h) in mm; not a paper name known to the PDF library
+    for unsupported in ( 'a5', 'a3', 'tabloid', '' ):
+        with pytest.raises( argparse.ArgumentTypeError ) as exc:
+            paper_format( unsupported )
+        assert "Unsupported paper size" in str( exc.value )
+        assert all( paper in str( exc.value ) for paper in PAPER_FORMATS )
+
+
+def test_main_paper( tmp_path, monkeypatch, capsys ):
+    monkeypatch.chdir( tmp_path )
+    # An unsupported paper size is refused up front, w/ a one-line message
+    with pytest.raises( SystemExit ) as exc:
+        main( [ '-q', '-c', 'BTC', '--paper', 'a5', 'Test' ] )
+    assert exc.value.code == 2
+    assert "Unsupported paper size 'a5'" in capsys.readouterr().err
+    assert not list( tmp_path.glob( '*.pdf' ))
+
+    # Supported sizes are produced
+    for paper in ( 'a4', 'Photo' ):
+        assert main( [ '-q', '-c', 'BTC', '--paper', paper, '-o', f"paper-{paper}", 'Test' ] ) == 0
+        assert ( tmp_path / f"paper-{paper}.pdf" ).stat().st_size > 0