Set min contig size to 2000 and warn if smaller contig is seen

We sometimes see users run Vamb with a lot of small contigs. In order to alert
users that this will results in degraded binning performance, Vamb now warns
users both with a runtime warning to stderr, and with a warning in the log file,
if any contigs are shorter than the minimum contig size.
Also, set the default minimum contig size to 2000.

test/test_parsecontigs.py

@@ -51,6 +51,10 @@ def test_warns_n_contigs(self):
         with self.assertWarns(UserWarning):
             Composition.from_file(self.io, minlength=250)
 
+    def test_warns_minlength(self):
+        with self.assertWarns(UserWarning):
+            Composition.from_file(self.large_io, minlength=275)
+
     def test_filter_minlength(self):
         minlen = 500
 

vamb/__main__.py

@@ -1790,8 +1790,8 @@ def add_input_output_arguments(subparser):
         dest="minlength",
         metavar="",
         type=int,
-        default=250,
-        help="ignore contigs shorter than this [250]",
+        default=2000,
+        help="ignore contigs shorter than this [2000]",
     )
     inputos.add_argument(
         "-z",

vamb/parsecontigs.py

@@ -5,7 +5,6 @@
 ...     tnfs, contignames, lengths = read_contigs(filehandle)
 """
 
-import sys
 import os as _os
 import numpy as _np
 import vamb.vambtools as _vambtools
@@ -163,14 +162,15 @@ def _convert(raw: _vambtools.PushArray, projected: _vambtools.PushArray):
     def from_file(
         cls: type[C],
         filehandle: Iterable[bytes],
-        minlength: int = 100,
+        minlength: int = 2000,
         logfile: Optional[IO[str]] = None,
     ) -> C:
         """Parses a FASTA file open in binary reading mode, returning Composition.
 
         Input:
             filehandle: Filehandle open in binary mode of a FASTA file
-            minlength: Ignore any references shorter than N bases [100]
+            minlength: Ignore any references shorter than N bases [2000]
+            logfile: Logfile to print warning to, if any
         """
 
         if minlength < 4:
@@ -181,11 +181,14 @@ def from_file(
         lengths = _vambtools.PushArray(_np.int32)
         mask = bytearray()  # we convert to Numpy at end
         contignames: list[str] = list()
+        minimum_seen_length = 2_000_000_000
 
         entries = _vambtools.byte_iterfasta(filehandle)
 
         for entry in entries:
-            skip = len(entry) < minlength
+            length = len(entry)
+            minimum_seen_length = min(minimum_seen_length, length)
+            skip = length < minlength
             mask.append(not skip)
 
             if skip:
@@ -227,4 +230,19 @@ def from_file(
                 print("\n", file=logfile)
                 print(message, file=logfile)
 
+
+        # Warn the user if any contigs have been observed, which is smaller
+        # than the threshold.
+        if not _np.all(metadata.mask):
+            message = (
+                f"WARNING: The minimum sequence length has been set to {minlength}, but a contig with "
+                f"length {minimum_seen_length} was seen. "
+                "Better results are obtained if the sequence file is filtered to the minimum "
+                "sequence length before mapping."
+            )
+            warnings.warn(message)
+            if logfile is not None:
+                print("\n", file=logfile)
+                print(message, file=logfile)
+
         return cls(metadata, tnfs_arr)