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Introduction to SearchIO

Matching the names in BioPerl, Biopython has a SeqIO module for sequence file input/output, and AlignIO for multiple sequence alignment input/output. The third member of the BioPerl trio is SearchIO, and a Biopython equivalent was written during summer 2012 by Google Summer of Code student Wibowo Arindrarto (blog).

This covers pairwise sequence search file input/output, for example from BLAST, HMMER, BLAT, or Bill Pearson’s FASTA suite. See the BioPerl SearchIO HOWTO for background.

Bio.SearchIO was included in Biopython 1.61 onwards as an experimental module, with that caveat dropped in Biopython 1.72.

There is a full chapter in the Tutorial (PDF).

This wiki describes the important bits with some small examples. For a full reference, consult the API documentation.

Supported File Formats

The table below lists all formats supported by Bio.SearchIO. Note that for writing support, the writer assumes that all the necessary attributes of the objects being written are present. It is not possible, for example, to write BLAST XML data to a HMMER 3.0 plain text output straight away.

Format name Read Write Index Notes
blast-tab 1.61 1.61 1.61 BLAST+ tabular output (both -m 6 and -m 7 flags are supported).
blast-text 1.61 n/a n/a BLAST+ plain text output (up to version 2.2.26+). Newer versions may not always work.
blast-xml 1.61 1.61 1.61 BLAST+ XML output.
blat-psl 1.61 1.61 1.61 BLAT default output (PSL format). Variants with or without header are both supported. PSLX (PSL + sequences) is also supported.
exonerate-text 1.61 n/a 1.61 Exonerate plain text output. Due to the way Biopython stores its sequences, at the moment support is limited to text outputs without split codons (for protein queries). If you are parsing a text output of protein queries containing split codon alignments (for example, from the protein2genome alignment mode), the parser will fail.
exonerate-cigar 1.61 n/a 1.61 Exonerate cigar string.
exonerate-vulgar 1.61 n/a 1.61 Exonerate vulgar string.
fasta-m10 1.61 n/a 1.61 Bill Pearson’s FASTA -m 10 output.
hmmer3-domtab 1.61 1.61 1.61 HMMER3.0 domain table output format. The name hmmer3-domtab per se is in fact not used, since the program name has to be specified. For example, when parsing hmmscan output, the format name would be hmmscan-domtab.
hmmer3-tab 1.61 1.61 1.61 HMMER 3.0 table output format.
hmmer3-text 1.61 n/a 1.61 HMMER 3.0 plain text output format.
hmmer2-text 1.61 n/a 1.61 HMMER 2.x plain text output format.
interproscan-xml 1.71 n/a n/a InterProScan XML output.

Format-specific Arguments

Although mostly similar to Biopython’s SeqIO and AlignIO modules, there is a small difference in the main Bio.SearchIO functions. Depending on the file format being used, you may pass additional keyword arguments that determines how the parser / indexer / writer behaves. Shown below are some formats which accepts extra keyword arguments.

Format name Argument name Default value Applicable for Explanation  
blast-tab comments False Reading, writing, indexing Boolean, whether the input/output file is the commented variant or not.  
  fields Default BLAST tabular output field names reading, writing, indexing Space-separated string, list of fields / columns in the input/output file.  
blast-xml encoding “utf-8” Writing XML encoding name.  
  indent ” “ (empty space) writing Character(s) to use for indenting the XML.  
  increment 2 writing How many times the character defined in indent are printed when printing a child element.  
blat-psl pslx False Reading, writing, indexing Boolean, whether the input/output file contains sequences or not.  
  header False writing Boolean, whether to write PSL header or not.  


The main goal of creating Bio.SearchIO is to have a common, easy to use interface across different search output files. As such, we have also created some conventions / standards for Bio.SearchIO that extend beyond the common object model. These conventions apply to all files parsed by Bio.SearchIO, regardless of their individual formats.

Python-style sequence coordinates

When storing sequence coordinates (start and end values), Bio.SearchIO uses the Python-style slice convention: zero-based and half-open intervals. For example, if in a BLAST XML output file the start and end coordinates of an HSP are 10 and 28, they would become 9 and 28 in Bio.SearchIO. The start coordinate becomes 9 because Python indices start from zero, while the end coordinate remains 28 as Python slices omit the last item in an interval.

Beside giving you the benefits of standardization, this convention also makes the coordinates usable for slicing sequences. For example, given a full query sequence and the start and end coordinates of an HSP, one can use the coordinates to extract part of the query sequence that results in the database hit.

When these objects are written to an output file using Bio.SearchIO.write, the coordinate values are restored to their respective format’s convention. Using the example above, if the HSP would be written to an XML file, the start and end coordinates would become 10 and 28 again.

Sequence coordinate order

Some search output format reverses the start and end coordinate sequences according to the sequence’s strand. For example, in BLAST plain text format if the matching strand lies in the minus orientation, then the start coordinate will always be bigger than the end coordinate.

In Bio.SearchIO, start coordinates are always smaller than the end coordinates, regardless of their originating strand. This ensures consistency when using the coordinates to slice full sequences.

Note that this coordinate order convention is only enforced in the HSPFragment level. If an HSP object has several HSPFragment objects, each individual fragment will conform to this convention. But the order of the fragments within the HSP object follows what the search output file uses.

Similar to the coordinate style convention, the start and end coordinates’ order are restored to their respective formats when the objects are written using Bio.SearchIO.write.

Frames and strand values

Bio.SearchIO only allows -1, 0, 1 and None as strand values. For frames, the only allowed values are integers from -3 to 3 (inclusive) and None. Both of these are standard Biopython conventions.