changeset 1:f3d024fd9bf4 draft

Uploaded python script

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/MDtag_filter.py	Tue Apr 24 12:14:56 2012 -0400
@@ -0,0 +1,101 @@
+#!/usr/bin/env python
+# Boris Rebolledo Jaramillo
+# [e-mail]
+# v1.0.2
+
+"""Filter mapped reads on MD tag string.
+
+This tool reads the MD tag of mapped reads (see SAM format specification).
+The user defines the 5' and 3' windows n and m (in bp), respectively.
+The mapped read is discarded if it contains any number of mismatches within n
+bases of the read 5' end and within m bases of the read 3' end.
+Option: save discarded reads in an additional SAM file.
+
+usage: %prog in.sam n m out.sam saveDiscarded?[yes/no] [discarded.sam]
+
+"""
+
+import sys,re
+
+if len(sys.argv) < 6:
+    sys.exit('ERROR! Usage: %prog <in.sam> <n> <m> <out.sam> '
+            '<saveDiscarded?[yes/no]> [<discarded.sam>]')
+if sys.argv[5] == 'yes' and len(sys.argv) < 7:
+    sys.exit('ERROR! Usage: %prog <in.sam> <n> <m> <out.sam> '
+            '<saveDiscarded?[yes/no]> [<discarded.sam>]')      
+      
+sam_file=list(open(sys.argv[1]))
+sam_mdfiltered=open(sys.argv[4],'w+')
+if sys.argv[5] == 'yes':
+    sam_discarded=open(sys.argv[6],'w+')
+
+
+# The MD tag is generated out of the alignment operations, regardless the strand
+# the read was mapped to. It describes the alignment from the leftmost aligned
+# base, which might be either the 5' or 3' end of the original read.
+# Luckily, the read orientation in the alignment can be extracted from the
+# alignment flag.
+# The code used here to identify the mapped reads orientation was taken from
+# the 'explain_sam_flags.py' script of the Picard-Tools.
+# It defines the meaning of the sam flags in plain English, so they can be used
+# to identify reads mapped to the reverse strand: flag = 16
+# http://picard-tools.sourcearchive.com/documentation/1.25-1/
+
+lstFlags = [
+    ("read paired", 0x1),
+    ("read mapped in proper pair", 0x2),
+    ("read unmapped", 0x4),
+    ("mate unmapped", 0x8),
+    ("read reverse strand", 0x10),
+    ("mate reverse strand", 0x20),
+    ("first in pair", 0x40),
+    ("second in pair", 0x80),
+    ("not primary alignment", 0x100),
+    ("read fails platform/vendor quality checks", 0x200),
+    ("read is PCR or optical duplicate", 0x400)
+    ]
+
+for line in sam_file:
+    if line.split("\t")[0].startswith('@'):
+        if sys.argv[5] == 'yes':
+            sam_discarded.write(line)
+            sam_mdfiltered.write(line)
+        else:
+            sam_mdfiltered.write(line)
+    else:
+        if len(line.split("\t")) > 11:        # Any SAM optional fields? 
+            for field in line.split("\t")[11:]:
+                if re.match('MD:Z:[0-9]+(([A-Z]|\^[A-Z]+)[0-9]+)*', field):        # is MD tag field present?
+                    flag = int(line.split("\t")[1]) 
+                    for strFlagName, iMask in lstFlags:
+                        if flag & iMask:
+                            if iMask == 16: 
+                                strand = 'reverse'
+                                break
+                            else:
+                                strand = 'forward'
+                        else:        # flag = 0 is not part of the definitions,
+                                     # but it corresponds to forward strand mapping
+                            strand = 'forward'
+                    # The position of the optional fields in the SAM format is variable. Finds the location of the MD tag:
+                    mdtag_idx = [i for i, item in enumerate(line.split("\t")[11:]) if re.match('MD:Z:[0-9]+(([A-Z]|\^[A-Z]+)[0-9]+)*', item)][0]
+                    mdtag=line.split("\t")[mdtag_idx+11].strip().split(":")[2]
+                    md_list = re.split(r'(\D)',mdtag)
+                    if strand == 'forward':
+                        if int(md_list[0]) >= int(sys.argv[2]) and int(md_list[-1]) >= int(sys.argv[3]):
+                            sam_mdfiltered.write(line)
+                        else:
+                            if sys.argv[5] == 'yes':
+                                sam_discarded.write(line)
+                    elif strand == 'reverse':
+                        if int(md_list[0]) >= int(sys.argv[3]) and int(md_list[-1]) >= int(sys.argv[2]):
+                            sam_mdfiltered.write(line)
+                        else:
+                            if sys.argv[5] == 'yes':
+                                sam_discarded.write(line)
+                    break
+
+sam_mdfiltered.close()
+
+if sys.argv[5] == 'yes':
+    sam_discarded.close()
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