Package Bio :: Package SearchIO :: Module BlatIO
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Module BlatIO

source code

Bio.SearchIO parser for BLAT output formats.

This module adds support for parsing BLAT outputs. BLAT (BLAST-Like Alignment Tool) is a sequence similarity search program initially built for annotating the human genome.

Bio.SearchIO.BlastIO was tested using standalone BLAT version 34, psLayout version 3. It should be able to parse psLayout version 4 without problems.

More information on BLAT is available from these sites:

Supported Formats

BlatIO supports parsing, indexing, and writing for both PSL and PSLX output formats, with or without header. To parse, index, or write PSLX files, use the 'pslx' keyword argument and set it to True.

# blat-psl defaults to PSL files >>> from Bio import SearchIO >>> psl = 'Blat/psl_34_004.psl' >>> qresult =, 'blat-psl') >>> qresult QueryResult(id='hg19_dna', 10 hits)

# set the pslx flag to parse PSLX files >>> pslx = 'Blat/pslx_34_004.pslx' >>> qresult =, 'blat-psl', pslx=True) >>> qresult QueryResult(id='hg19_dna', 10 hits)

For parsing and indexing, you do not need to specify whether the file has a header or not. For writing, if you want to write a header, you can set the 'header' keyword argument to True. This will write a 'psLayout version 3' header to your output file.

from Bio import SearchIO qresult =, 'blat-psl') SearchIO.write(qresult, 'header.psl', header=True) <stdout> (1, 10, 19, 23)

Note that the number of HSPFragments written may exceed the number of HSP objects. This is because in PSL files, it is possible to have single matches consisting of noncontiguous sequence fragments. This is where the HSPFragment object comes into play. These fragments are grouped into a single HSP because they share the same statistics (e.g. match numbers, BLAT score, etc.). However, they do not share the same sequence attributes, such as the start and end coordinates, making them distinct objects.

In addition to parsing PSL(X) files, BlatIO also computes the percent identities and scores of your search results. This is done using the calculation formula posted here: It mimics the score and percent identity calculation done by UCSC's web BLAT service.

Since BlatIO parses the file in a single pass, it expects all results from the same query to be in consecutive rows. If the results from one query are spread in nonconsecutive rows, BlatIO will consider them to be separate QueryResult objects.

In most cases, the PSL(X) format uses the same coordinate system as Python (zero-based, half open). These coordinates are anchored on the plus strand. However, if the query aligns on the minus strand, BLAT will anchor the qStarts coordinates on the minus strand instead. BlatIO is aware of this, and will re-anchor the qStarts coordinates to the plus strand whenever it sees a minus strand query match. Conversely, when you write out to a PSL(X) file, BlatIO will reanchor qStarts to the minus strand again.

BlatIO provides the following attribute-column mapping:

Object Attribute Column Name, Value
QueryResutl id Q name, query sequence ID
seq_len Q size, query sequence full length
Hit id T name, hit sequence ID
seq_len T size, hit sequence full length
HSP hit_end T end, end coordinate of the last hit fragment
hit_gap_num T gap bases, number of bases inserted in hit
hit_gapopen_num T gap count, number of hit gap inserts
hit_span_all blockSizes, sizes of each fragment
hit_start T start, start coordinate of the first hit fragment
hit_start_all tStarts, start coordinate of each hit fragment
match_num match, number of non-repeat matches
mismatch_num mismatch, number of mismatches
match_rep_num rep. match, number of matches that are part of repeats
n_num N's, number of N bases
query_end Q end, end coordinate of the last
query_gap_num query fragment Q gap bases, number of bases inserted in query
query_gapopen_num Q gap count, number of query gap inserts
query_span_all blockSizes, sizes of each fragment
query_start Q start, start coordinate of the first query block
query_start_all qStarts, start coordinate of each query fragment
len [1] block count, the number of blocks in the alignment
HSPFragment hit hit sequence, if present
hit_strand strand, hit sequence strand
query query sequence, if present
query_strand strand, query sequence strand

In addition to the column mappings above, BlatIO also provides the following object attributes:

Object Attribute Value
HSP gapopen_num
Q gap count + T gap count, total
number of gap openings
ident_num matches + repmatches, total number of identical residues
ident_pct percent identity, calculated using UCSC's formula
query_is_protein boolean, whether the query sequence is a protein
score HSP score, calculated using UCSC's formula

Finally, the default HSP and HSPFragment properties are also provided. See the HSP and HSPFragment documentation for more details on these properties.

[1]You can obtain the number of blocks / fragments in the HSP by invoking len on the HSP
Classes [hide private]
Parser for the BLAT PSL format.
Indexer class for BLAT PSL output.
Writer for the blat-psl format.
Functions [hide private]
_list_from_csv(csv_string, caster=None)
Transforms the given comma-separated string into a list.
source code
_reorient_starts(starts, blksizes, seqlen, strand)
Reorients block starts into the opposite strand's coordinates.
source code
_is_protein(psl) source code
_calc_millibad(psl, is_protein) source code
_calc_score(psl, is_protein) source code
_create_hsp(hid, qid, psl) source code
Variables [hide private]
  _PTR_ROW_CHECK = '^\\d+\\s+\\d+\\s+\\d+\\s+\\d+'
  _RE_ROW_CHECK = re.compile(r'^\d+\s+\d+\s+\d+\s+\d+')
  _RE_ROW_CHECK_IDX = re.compile(r'^\d+\s+\d+\s+\d+\s+\d+')
  __package__ = 'Bio.SearchIO'
Function Details [hide private]

_list_from_csv(csv_string, caster=None)

source code 
Transforms the given comma-separated string into a list.
  • csv_string (string) - comma-separated input string
  • caster (callable, accepts string, returns object) - function used to cast each item in the input string to its intended type

_reorient_starts(starts, blksizes, seqlen, strand)

source code 
Reorients block starts into the opposite strand's coordinates.
  • starts (list [int]) - start coordinates
  • blksizes (list [int]) - block sizes
  • seqlen (int) - sequence length
  • strand (int, choice of -1, 0, or 1) - sequence strand