Refactor logic to read trees from multiple formats into a function

augur/ancestral.py

@@ -6,7 +6,7 @@
 from Bio import Phylo, SeqIO
 from Bio.Seq import Seq
 from Bio.SeqRecord import SeqRecord
-from .utils import write_json
+from .utils import read_tree, InvalidTreeError, write_json
 from treetime.vcf_utils import read_vcf, write_vcf
 from collections import defaultdict
 
@@ -106,15 +106,11 @@ def run(args):
     is_vcf = False
     ref = None
     anc_seqs = {}
-    # check if tree is provided and can be read
-    for fmt in ["newick", "nexus"]:
-        try:
-            T = Phylo.read(args.tree, fmt)
-            break
-        except:
-            pass
-    if T is None:
-        print("ERROR: reading tree from %s failed."%args.tree)
+
+    try:
+        T = read_tree(args.tree)
+    except (FileNotFoundError, InvalidTreeError) as error:
+        print("ERROR: %s" % error, file=sys.stderr)
         return 1
 
     import numpy as np

augur/refine.py

@@ -4,9 +4,10 @@
 
 import os, shutil, time, sys
 from Bio import Phylo
-from .utils import read_metadata, get_numerical_dates, write_json
+from .utils import read_metadata, read_tree, get_numerical_dates, write_json, InvalidTreeError
 from treetime.vcf_utils import read_vcf, write_vcf
 
+
 def refine(tree=None, aln=None, ref=None, dates=None, branch_length_inference='auto',
              confidence=False, resolve_polytomies=True, max_iter=2,
              infer_gtr=True, Tc=0.01, reroot=None, use_marginal=False, fixed_pi=None,
@@ -127,18 +128,11 @@ def run(args):
     # list of node attributes that are to be exported, will grow
     attributes = ['branch_length']
 
-    # check if tree is provided an can be read
-    T = None #otherwise get 'referenced before assignment' error if reading fails
-    for fmt in ["newick", "nexus"]:
-        try:
-            T = Phylo.read(args.tree, fmt)
-            node_data['input_tree'] = args.tree
-            break
-        except Exception as error:
-            print("\n\nERROR: reading tree from %s failed: %s" % (args.tree, error))
-            return 1
-    if T is None:
-        print("\n\nERROR: reading tree from %s failed."%args.tree)
+    try:
+        T = read_tree(args.tree)
+        node_data['input_tree'] = args.tree
+    except (FileNotFoundError, InvalidTreeError) as error:
+        print("ERROR: %s" % error, file=sys.stderr)
         return 1
 
     if not args.alignment:

augur/utils.py

@@ -93,6 +93,64 @@ def get_numerical_dates(meta_dict, name_col = None, date_col='date', fmt=None, m
 
     return numerical_dates
 
+
+class InvalidTreeError(Exception):
+    """Represents an error loading a phylogenetic tree from a filename.
+    """
+    pass
+
+
+def read_tree(fname, min_terminals=3):
+    """Safely load a tree from a given filename or raise an error if the file does
+    not contain a valid tree.
+
+    Parameters
+    ----------
+    fname : str
+        name of a file containing a phylogenetic tree
+
+    min_terminals : int
+        minimum number of terminals required for the parsed tree as a sanity
+        check on the tree
+
+    Raises
+    ------
+    InvalidTreeError
+        If the given file exists but does not seem to contain a valid tree format.
+
+    Returns
+    -------
+    Bio.Phylo :
+        BioPython tree instance
+
+    """
+    T = None
+    supported_tree_formats = ["newick", "nexus"]
+    for fmt in supported_tree_formats:
+        try:
+            T = Bio.Phylo.read(fname, fmt)
+
+            # Check the sanity of the parsed tree to handle cases when non-tree
+            # data are still successfully parsed by BioPython. Too few terminals
+            # in a tree indicates that the input is not valid.
+            if T.count_terminals() < min_terminals:
+                T = None
+            else:
+                break
+        except ValueError:
+            # We cannot open the tree in the current format, so we will try
+            # another.
+            pass
+
+    # If the tree cannot be loaded, raise an error to that effect.
+    if T is None:
+        raise InvalidTreeError(
+            "Could not read the given tree %s using the following supported formats: %s" % (fname, ", ".join(supported_tree_formats))
+        )
+
+    return T
+
+
 def read_node_data(fnames, tree=None):
     """parse the "nodes" field of the given JSONs and join the data together"""
     if type(fnames) is str: