Esempio n. 1
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def calculate_snp_distances(options_dict):
    """Calculate pairwise sample SNP distances.

    Description:
    Calculate pairwise SNP distances from the multi-fasta SNP matrix.
    Generate a file of pairwise distances and a file containing a matrix
    of distances.
    This function expects, or creates '(*)', the following files:
            snpma.fasta
            snp_distance_pairwise.tsv*
            snp_distance_matrix.tsv*

    The files are used as follows:
        1. The snpma.fasta input file contains the snp matrix for all samples
        2. The snp_distance_pairwise.tsv output file contains a three column
            tab-separated table of distances between all pairs of samples
        2. The snp_distance_matrix.tsv output file contains a matrix of
           distances between all samples.

    Args:
        inputFile: File path (not just file name) for the snp matrix in fasta format
        pairwiseFile: File path (not just file name) of the output pairwise distance file
        matrixFile: File path (not just file name) for the output distance matrix file

    Raises:

    Examples:
    options_dict = {'inputFile':'snpma.fasta',
                    'pairwiseFile':'snp_distance_pairwise.tsv',
                    'matrixFile':'snp_distance_matrix.tsv'
                   }
    calculate_snp_distances(options_dict)
    """
    print_log_header()
    verbose_print("# %s %s" % (utils.timestamp(), utils.command_line_short()))
    verbose_print("# %s version %s" % (utils.program_name(), __version__))
    print_arguments(options_dict)

    #==========================================================================
    # Validate arguments
    #==========================================================================
    input_file = options_dict['inputFile']
    pairwise_file = options_dict['pairwiseFile']
    matrix_file = options_dict['matrixFile']
    force_flag = options_dict['forceFlag']

    bad_file_count = utils.verify_existing_input_files("SNP matrix file", [input_file])
    if bad_file_count > 0:
        utils.global_error("Error: cannot calculate sequence distances without the snp matrix file.")

    if not pairwise_file and not matrix_file:
        utils.global_error("Error: no output file specified.")

    #==========================================================================
    # Check freshness
    #==========================================================================
    rebuild_pairwise_file = pairwise_file and utils.target_needs_rebuild([input_file], pairwise_file)
    rebuild_matrix_file = matrix_file and utils.target_needs_rebuild([input_file], matrix_file)
    if force_flag or rebuild_pairwise_file or rebuild_matrix_file:

        #------------------------------
        # Read in snp matrix file
        #------------------------------
        seqs = {}
        with open(input_file) as ifile:
            for line in ifile:
                line = line.rstrip('\n')
                if line.startswith('>'):
                    curr_sample = line.lstrip('>')
                    seqs[curr_sample] = ''
                else:
                    seqs[curr_sample] += str(line)

        #------------------------------
        # Count mismatches
        #------------------------------
        ids = sorted(seqs.keys())
        pairwise_mismatches = dict() # tuple (seq1 id, seq2 id) -> int

        for id1, id2 in itertools.combinations(ids, 2):
            mismatches = utils.calculate_sequence_distance(seqs[id1], seqs[id2])
            pairwise_mismatches[(id1, id2)] = mismatches
            pairwise_mismatches[(id2, id1)] = mismatches

        #------------------------------
        # Print distance files
        #------------------------------
        if pairwise_file:
            with open(pairwise_file, 'w') as p_out:
                p_out.write('%s\n' % '\t'.join(['Seq1', 'Seq2', 'Distance']))
                for id1, id2 in itertools.product(ids, ids):
                    mismatches = pairwise_mismatches.get((id1, id2), 0) # zero when id1=id2
                    p_out.write("%s\t%s\t%i\n" % (id1, id2, mismatches))

        if matrix_file:
            with open(matrix_file, 'w') as m_out:
                m_out.write('\t%s\n' % '\t'.join(ids)) # matrix header
                # write table of mismatches
                for id1 in ids:
                    mismatches = [pairwise_mismatches.get((id1, id2), 0) for id2 in ids]
                    mismatch_strs = map(str, mismatches)
                    m_out.write("%s\t%s\n" % (id1, '\t'.join(mismatch_strs)))

    else:
        verbose_print("Distance files have already been freshly built.  Use the -f option to force a rebuild.")
    verbose_print("# %s %s finished" % (utils.timestamp(), utils.program_name()))
Esempio n. 2
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def create_snp_list(options_dict):
    """Create SNP list file

    Description:
    Create the SNP list -- the list of positions where variants were found
    and the corresponding list of samples having a variant at each position.
    This function expects, or creates '(*)', the following files arranged
    in the following way:
            sampleDirectories.txt
            samples
                sample_name_one/var.flt.vcf
                ...
            snplist.txt (*)

    The files are used as follows:
        1. The sampleDirectories.txt input file contains a list of the paths to
           the sample directories.
        2. The var.flt.vcf variant input files are used to construct the
           SNP position list.
        3. The snplist.txt output file contains the union of the SNP positions
           and sample names extracted from all the var.flt.vcf files.

    The sampleDirectories.txt and var.flt.vcf files are created outside of
    this function. The package documentation provides an example of creating
    these files based on the lambda_virus sequence that is used as one test
    for this package.

    Args:
        sampleDirsFile: File path (not just file name) of file containing paths
            to directories containing var.flt.vcf file for each sequence.
        vcfFileName: File name of the VCF files which must exist in each of the
            sample directories
        snpListFile: File path (not just file name) of text format list
            of SNP positions

    Raises:

    Examples:
    options_dict = {'sampleDirsFile':'sampleDirectories.txt',
                    'vcfFileName':'var.flt.vcf'
                    'snpListFile':'snplist.txt',
                   }
    create_snp_list(options_dict)
    """
    print_log_header()
    verbose_print("# %s %s" % (utils.timestamp(), utils.command_line_short()))
    verbose_print("# %s version %s" % (utils.program_name(), __version__))
    print_arguments(options_dict)

    #==========================================================================
    # Prep work
    #==========================================================================
    sample_directories_list_path = options_dict['sampleDirsFile']
    bad_file_count = utils.verify_non_empty_input_files("File of sample directories", [sample_directories_list_path])
    if bad_file_count > 0:
        utils.global_error(None)

    with open(sample_directories_list_path, "r") as sample_directories_list_file:
        unsorted_list_of_sample_directories = [line.rstrip() for line in sample_directories_list_file]
    unsorted_list_of_sample_directories = [d for d in unsorted_list_of_sample_directories if d]
    sorted_list_of_sample_directories = sorted(unsorted_list_of_sample_directories)

    #==========================================================================
    # Validate inputs
    #==========================================================================
    snp_list_file_path = options_dict['snpListFile']
    vcf_file_name = options_dict['vcfFileName']
    list_of_vcf_files = [os.path.join(dir, vcf_file_name) for dir in sorted_list_of_sample_directories]

    bad_file_count = utils.verify_non_empty_input_files("VCF file", list_of_vcf_files)
    if bad_file_count == len(list_of_vcf_files):
        utils.global_error("Error: all %d VCF files were missing or empty." % bad_file_count)
    elif bad_file_count > 0:
        utils.sample_error("Error: %d VCF files were missing or empty." % bad_file_count, continue_possible=True)

    #==========================================================================
    # Read in all vcf files and process into dict of SNPs passing various
    # criteria. Do this for each sample. Write to file.
    #==========================================================================
    if options_dict['forceFlag'] or utils.target_needs_rebuild(list_of_vcf_files, snp_list_file_path):
        snp_dict = dict()
        excluded_sample_directories = set()
        for sample_dir, vcf_file_path in zip(sorted_list_of_sample_directories, list_of_vcf_files):

            if not os.path.isfile(vcf_file_path):
                continue
            if os.path.getsize(vcf_file_path) == 0:
                continue

            verbose_print("Processing VCF file %s" % vcf_file_path)
            sample_name = os.path.basename(os.path.dirname(vcf_file_path))
            snp_set = utils.convert_vcf_file_to_snp_set(vcf_file_path)
            max_snps = options_dict['maxSnps']
            if max_snps >= 0 and len(snp_set) > max_snps:
                verbose_print("Excluding sample %s having %d snps." % (sample_name, len(snp_set)))
                excluded_sample_directories.add(sample_dir)
                continue

            for key in snp_set:
                if key not in snp_dict:
                    sample_list = [sample_name]
                    snp_dict[key] = sample_list
                else:
                    sample_list = snp_dict[key]
                    sample_list.append(sample_name)

        verbose_print('Found %d snp positions across %d sample vcf files.' % (len(snp_dict), len(list_of_vcf_files)))
        utils.write_list_of_snps(snp_list_file_path, snp_dict)
        verbose_print("")

        #==========================================================================
        # Write the filtered list of sample directories
        #==========================================================================
        sample_directories_list_path = sample_directories_list_path + ".filtered"
        with open(sample_directories_list_path, "w") as filtered_samples_file_object:
            # Loop over the unsorted list to keep the order of samples the same as the original.
            # This will keep the same HPC log file suffix number.
            for sample_dir in unsorted_list_of_sample_directories:
                if sample_dir not in excluded_sample_directories:
                    filtered_samples_file_object.write("%s\n" % sample_dir)
    else:
        verbose_print("SNP list %s has already been freshly built.  Use the -f option to force a rebuild." % snp_list_file_path)
    verbose_print("# %s %s finished" % (utils.timestamp(), utils.program_name()))
Esempio n. 3
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def create_snp_reference_seq(options_dict):
    """Write reference sequence bases at SNP locations to a fasta file.

    Description:
    Write reference sequence bases at SNP locations to a fasta file.
    This function expects, or creates '(*)', the following files:
            reference.fasta
            snplist.txt
            referenceSNP.fasta (*)

    The files are used as follows:
        1. The reference.fasta input file contains the whole-genome reference
           bases.
        2. The snplist.txt input file contains the list of SNP positions across
           all the samples.
        2. The referenceSNP.fasta output file contains the reference bases at
           the identified SNP locations.

    The snplist.txt file is created outside of this function.  The package
        documentation provides an example of creating this file based on the
        lambda_virus sequence that is used as one test for this package.

    Args:
        referenceFile: File path (not just file name) for reference sequence
            (in fasta format
        snpListFile: File path (not just file name) of text format list of SNP
            positions
        snpRefFile: File path (not just file name) for the SNP reference
            sequence file.

    Raises:

    Examples:
    options_dict = {'referenceFile':'reference.fasta',
                    'snpListFile':'snplist.txt',
                    'snpRefFile':'referenceSNP.fasta'
                   }
    create_snp_reference_seq(options_dict)
    """
    print_log_header()
    verbose_print("# %s %s" % (utils.timestamp(), utils.command_line_short()))
    verbose_print("# %s version %s" % (utils.program_name(), __version__))
    print_arguments(options_dict)

    #==========================================================================
    #    Write reference sequence bases at SNP locations to a fasta file.
    #==========================================================================
    reference_file = options_dict['referenceFile']
    snp_list_file_path = options_dict['snpListFile']
    snp_ref_seq_path = options_dict['snpRefFile']

    #==========================================================================
    # Verify input files exist
    #==========================================================================
    bad_file_count = utils.verify_existing_input_files("Snplist file", [snp_list_file_path])
    if bad_file_count > 0:
        utils.global_error("Error: cannot create the snp reference sequence without the snplist file.")

    bad_file_count = utils.verify_non_empty_input_files("Reference file", [reference_file])
    if bad_file_count > 0:
        utils.global_error("Error: cannot create the snp reference sequence without the reference fasta file.")

    #==========================================================================
    # Find the reference bases at the snp positions
    #==========================================================================
    source_files = [reference_file, snp_list_file_path]
    if options_dict['forceFlag'] or utils.target_needs_rebuild(source_files, snp_ref_seq_path):
        utils.write_reference_snp_file(reference_file, snp_list_file_path, snp_ref_seq_path)
        verbose_print("")
    else:
        verbose_print("SNP reference sequence %s has already been freshly built.  Use the -f option to force a rebuild." % snp_ref_seq_path)

    verbose_print("# %s %s finished" % (utils.timestamp(), utils.program_name()))
Esempio n. 4
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def create_snp_matrix(options_dict):
    """Create SNP matrix

    Description:
    Create the SNP matrix containing the consensus base for each of the samples
    at the positions where SNPs were found in any of the samples.  The matrix
    contains one row per sample and one column per SNP position.  Non-SNP
    positions are not included in the matrix.
    This function expects, or creates '(*)', the following
        files arranged in the following way:
            sampleDirectories.txt
            samples
                sample_name_one/consensus.fasta
                ...
            snpma.fasta (*)

    The files are used as follows:
        1. The sampleDirectories.txt input file contains a list of the paths to
           the sample directories.
        2. The consensus.fasta input files are previously called consensus
           for each sample to construct the SNP matrix fasta file.
        3. The snpma.fasta output file contains the SNP calls for each
           sequence, arranged as a multi-fasta file with one sequence per
           sample.

    The sampleDirectories.txt, and consensus.fasta are created outside of this
        function. The package documentation provides an example of creating
        these files based on the lambda_virus sequence that is used as one
        test for this package.

    Args:
        sampleDirsFile : str
            File path (not just file name) of file containing paths
            to directories containing consensus.fasta file for each sequence.
        snpListFile : str
            File path (not just file name) of text format list of SNP positions
        consFileName : str
            File name of the previously called consensus fasta files which must
            exist in each of the sample directories
        snpmaFile : str
            File path (not just file name) of the output snp matrix, formatted
            as a fasta file, with each sequence (all of identical length)
            corresponding to the SNPs in the correspondingly named sequence.

    Raises:

    Examples:
    options_dict = {'sampleDirsFile':'sampleDirectories.txt',
                    'consFileName':'consensus.fasta',
                    'snpmaFile':'snpma.fasta',
                    'minConsFreq':0.6,
                   }
    create_snp_matrix(options_dict)
    """
    print_log_header()
    verbose_print("# %s %s" % (utils.timestamp(), utils.command_line_short()))
    verbose_print("# %s version %s" % (utils.program_name(), __version__))
    print_arguments(options_dict)

    #==========================================================================
    # Prep work
    #==========================================================================
    sample_directories_list_filename = options_dict['sampleDirsFile']
    bad_file_count = utils.verify_non_empty_input_files("File of sample directories", [sample_directories_list_filename])
    if bad_file_count > 0:
        utils.global_error(None)

    with open(sample_directories_list_filename, "r") as sample_directories_list_file:
        list_of_sample_directories = [line.rstrip() for line in sample_directories_list_file]
    list_of_sample_directories = sorted([d for d in list_of_sample_directories if d])

    #==========================================================================
    # Verify input consensus.fasta files exist
    #==========================================================================
    consensus_files = []
    bad_file_count = 0
    for sample_directory in list_of_sample_directories:
        consensus_file_path = os.path.join(sample_directory, options_dict['consFileName'])
        bad_count = utils.verify_non_empty_input_files("Consensus fasta file", [consensus_file_path])
        if bad_count == 1:
            bad_file_count += 1
        else:
            consensus_files.append(consensus_file_path)  # keep the list of good files

    if bad_file_count == len(list_of_sample_directories):
        utils.global_error("Error: all %d consensus fasta files were missing or empty." % bad_file_count)
    elif bad_file_count > 0:
        utils.sample_error("Error: %d consensus fasta files were missing or empty." % bad_file_count, continue_possible=True)

    #==========================================================================
    # Check if the result is already fresh
    #==========================================================================
    snpma_file_path = options_dict['snpmaFile']
    source_files = consensus_files
    if not options_dict['forceFlag']:
        if not utils.target_needs_rebuild(source_files, snpma_file_path):
            verbose_print("SNP matrix %s has already been freshly built.  Use the -f option to force a rebuild." % snpma_file_path)
            verbose_print("# %s %s finished" % (utils.timestamp(), utils.program_name()))
            return

    #==========================================================================
    #   Create snp matrix. Write results to file.
    #==========================================================================
    with open(snpma_file_path, "w") as output_file:
        for consensus_file_path in consensus_files:
            verbose_print("Merging " + consensus_file_path)
            with open(consensus_file_path, "r") as input_file:
                for line in input_file:
                    output_file.write(line)

    verbose_print("")
    verbose_print("# %s %s finished" % (utils.timestamp(), utils.program_name()))
Esempio n. 5
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def call_consensus(options_dict):
    """Call the consensus base for a sample

    Call the consensus base for a sample at the positions where SNPs were found
    in any of the samples.
    This function expects, or creates '(*)', the following
        files arranged in the following way:
            snplist.txt
            samples
                sample_name_one/reads.all.pileup
                sample_name_one/consensus.fasta (*)

    The files are used as follows:
        1. The snplist.txt input file contains the list of SNP positions
           extracted from all the var.flt.vcf files combined.
        2. The reads.all.pileup input file is a pileups at all positions
           used to determine the nucleotide base at each SNP position.
        3. The consensus.fasta output file contains the SNP calls for each
           sequence, arranged as a fasta file with one sequence per sample.

    The snplist.txt, and reads.snp.pileup are created outside of this function.
       The package documentation provides an example
        of creating these files based on the lambda_virus sequence that is used
        as one test for this package.

    Args:
        forceFlag : boolean
            flag to force processing even when result file already exists and
            is newer than inputs
        snpListFile : str
            File path (not just file name) of text format list of SNP positions
        allPileupFile : str
            Relative or absolute path to the genome-wide pileup file for this
            sample
        consensusFile : str
            Output file. Relative or absolute path to the consensus fasta file
            for this sample.
        minBaseQual : int
            Mimimum base quality score to count a read. All other snp filters
            take effect after the low-quality reads are discarded.
        minConsFreq : float
            Consensus frequency. Mimimum fraction of high-quality reads
            supporting the consensus to make a call.
        minConsStrdDpth : int
            Consensus strand depth. Minimum number of high-quality reads
            supporting the consensus which must be present on both the
            forward and reverse strands to make a call.
        minConsStrdBias : float
            Strand bias. Minimum fraction of the high-quality
            consensus-supporting reads which must be present on both the
            forward and reverse strands to make a call. The numerator of this
            fraction is the number of high-quality consensus-supporting reads
            on one strand.  The denominator is the total number of high-quality
            consensus-supporting reads on both strands combined.

    Raises:

    Examples:
    options_dict = {'snpListFile':'snplist.txt',
                    'allPileupFile':'reads.all.pileup',
                    'consensusFile':'consensus.fasta',
                    'minBaseQual':15,
                    'minConsFreq':0.6,
                    'minConsStrdDpth':4,
                    'minConsStrdBias':0.10,
                    'vcfFailedSnpGt':'.'
                   }
    call_consensus(options_dict)
    """
    print_log_header()
    verbose_print("# %s %s" % (utils.timestamp(), utils.command_line_short()))
    verbose_print("# %s version %s" % (utils.program_name(), __version__))
    print_arguments(options_dict)

    snp_list_file_path = options_dict['snpListFile']
    all_pileup_file_path = options_dict['allPileupFile']
    sample_directory = os.path.dirname(os.path.abspath(all_pileup_file_path))
    sample_name = os.path.basename(sample_directory)
    consensus_file_path = options_dict['consensusFile']
    consensus_file_dir = os.path.dirname(os.path.abspath(consensus_file_path))
    vcf_file_name = options_dict['vcfFileName']
    vcf_file_path = os.path.join(consensus_file_dir, vcf_file_name) if vcf_file_name else None

    bad_file_count = utils.verify_existing_input_files("Snplist file", [snp_list_file_path])
    if bad_file_count > 0:
        utils.global_error("Error: cannot call consensus without the snplist file.")

    bad_file_count = utils.verify_non_empty_input_files("Pileup file", [all_pileup_file_path])
    if bad_file_count > 0:
        utils.sample_error("Error: cannot call consensus without the pileup file.", continue_possible=False)

    # Check if the result is already fresh
    source_files = [snp_list_file_path, all_pileup_file_path]
    if not options_dict['forceFlag'] and not utils.target_needs_rebuild(source_files, consensus_file_path):
        verbose_print("Consensus call file %s has already been freshly built.  Use the -f option to force a rebuild." % consensus_file_path)
        verbose_print("# %s %s finished" % (utils.timestamp(), utils.program_name()))
        return

    # Load the list of which positions to called
    snp_list = utils.read_snp_position_list(snp_list_file_path)
    snplist_length = len(snp_list)
    verbose_print("snp position list length = %d" % snplist_length)

    # Call consensus. Write results to file.
    position_consensus_base_dict = dict()

    caller = pileup.ConsensusCaller(options_dict['minConsFreq'],
                                    options_dict['minConsStrdDpth'],
                                    options_dict['minConsStrdBias'])
    snp_positions = set(snp_list)
    parse_positions = None if options_dict['vcfAllPos'] else snp_positions
    pileup_reader = pileup.Reader(all_pileup_file_path,
                                  options_dict['minBaseQual'],
                                  parse_positions)
    if vcf_file_name:
        writer = vcf_writer.SingleSampleWriter(vcf_file_path, options_dict['vcfPreserveRefCase'])
        filters = caller.get_filter_descriptions()
        writer.write_header(sample_name, filters, options_dict['vcfRefName'])
    for pileup_record in pileup_reader:
        chrom = pileup_record.chrom
        pos = pileup_record.position
        consensus_base, fail_reasons = caller.call_consensus(pileup_record)
        if (chrom, pos) in snp_positions:
            if fail_reasons:
                position_consensus_base_dict[(chrom, pos)] = '-'
            else:
                position_consensus_base_dict[(chrom, pos)] = consensus_base

        if vcf_file_name:
            writer.write_from_pileup(pileup_record, fail_reasons, options_dict['vcfFailedSnpGt'])
    if vcf_file_name:
        writer.close()

    verbose_print("called consensus positions = %i" % (len(position_consensus_base_dict)))

    consensus_list = [position_consensus_base_dict.get(key, '-') for key in snp_list]
    consensus_str = ''.join(consensus_list)
    snp_seq_record = SeqRecord(Seq(consensus_str), id=sample_name, description="")

    # Write the consensus calls to a fasta file
    with open(consensus_file_path, "w") as fasta_file_object:
        SeqIO.write([snp_seq_record], fasta_file_object, "fasta")

    verbose_print("")
    verbose_print("# %s %s finished" % (utils.timestamp(), utils.program_name()))
Esempio n. 6
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def create_snp_pileup(options_dict):
    """Create the SNP pileup file for a sample.

    Description:
    Create the SNP pileup file for a sample -- the pileup file restricted to
    only positions where variants were found in any sample.
    This function expects, or creates '(*)', the following files arranged
    in the following way:
            snplist.txt
            samples
                sample_name_one/reads.all.pileup
                sample_name_one/reads.snp.pileup (*)
                ...

    The files are used as follows:
        1. The snplist.txt input file contains the list of SNP positions
           extracted from the var.flt.vcf file.
        2. The reads.all.pileup input file is the genome-wide pileup file
           for this sample.
        3. The reads.snp.pileup output file is the pileup file for this sample,
           restricted to only positions where variants were found in any
           sample.

    The snplist.txt and reads.all.pileup files are created outside of this
    function. The package documentation provides an example of creating these
    files based on the lambda_virus sequence that is used as one test for
    this package.

    Args:
        snpListFile: File path (not just file name) of text format list
            of SNP positions across all samples
        allPileupFile: File path (not just file name) of the whole-genome
            pileup file fot this sample
        snpPileupFile: File path (not just file name) of the snp pileup file

    Raises:

    Examples:
    options_dict = {'snpListFile':'snplist.txt',
                    'allPileupFile':'samples/SRR555888/reads.all.pileup'
                    'snpPileupFile':'samples/SRR555888/reads.snp.pileup'
                   }
    create_snp_pileup(options_dict)
    """
    print_log_header()
    verbose_print("# %s %s" % (utils.timestamp(), utils.command_line_short()))
    verbose_print("# %s version %s" % (utils.program_name(), __version__))
    print_arguments(options_dict)

    snp_list_file_path = options_dict['snpListFile']
    all_pileup_file_path = options_dict['allPileupFile']
    snp_pileup_file_path = options_dict['snpPileupFile']

    source_files = [snp_list_file_path, all_pileup_file_path]
    if options_dict['forceFlag'] or utils.target_needs_rebuild(source_files, snp_pileup_file_path):
        # Create a pileup file with a subset of the whole-genome pileup restricted
        # to locations with SNPs only.
        snp_list = utils.read_snp_position_list(snp_list_file_path)
        utils.create_snp_pileup(all_pileup_file_path, snp_pileup_file_path, set(snp_list))
        verbose_print("")
    else:
        verbose_print("SNP pileup %s has already been freshly built.  Use the -f option to force a rebuild." % snp_pileup_file_path)
    verbose_print("# %s %s finished" % (utils.timestamp(), utils.program_name()))
Esempio n. 7
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def create_snp_list(options_dict):
    """Create SNP list file

    Description:
    Create the SNP list -- the list of positions where variants were found
    and the corresponding list of samples having a variant at each position. 
    This function expects, or creates '(*)', the following files arranged 
    in the following way:
            sampleDirectories.txt
            samples
                sample_name_one/var.flt.vcf
                ...
            snplist.txt (*)

    The files are used as follows:
        1. The sampleDirectories.txt input file contains a list of the paths to 
           the sample directories.
        2. The var.flt.vcf variant input files are used to construct the 
           SNP position list.
        3. The snplist.txt output file contains the union of the SNP positions 
           and sample names extracted from all the var.flt.vcf files.

    The sampleDirectories.txt and var.flt.vcf files are created outside of 
    this function. The package documentation provides an example of creating 
    these files based on the lambda_virus sequence that is used as one test 
    for this package.

    Args:
        sampleDirsFile: File path (not just file name) of file containing paths 
            to directories containing var.flt.vcf file for each sequence.
        vcfFileName: File name of the VCF files which must exist in each of the
            sample directories
        snpListFile: File path (not just file name) of text format list 
            of SNP positions

    Raises:

    Examples:
    options_dict = {'sampleDirsFile':'sampleDirectories.txt',
                    'vcfFileName':'var.flt.vcf'
                    'snpListFile':'snplist.txt',
                   }
    create_snp_list(options_dict)
    """
    print_log_header()
    verbose_print("# %s %s" % (utils.timestamp(), utils.command_line_short()))
    verbose_print("# %s version %s" % (utils.program_name(), __version__))
    print_arguments(options_dict)

    #==========================================================================
    # Prep work
    #==========================================================================
    sample_directories_list_filename = options_dict['sampleDirsFile']
    bad_file_count = utils.verify_non_empty_input_files("File of sample directories", [sample_directories_list_filename])
    if bad_file_count > 0:
        utils.global_error(None)

    with open(sample_directories_list_filename, "r") as sample_directories_list_file:
        list_of_sample_directories = [line.rstrip() for line in sample_directories_list_file]
    list_of_sample_directories = sorted([d for d in list_of_sample_directories if d])

    #==========================================================================
    # Read in all vcf files and process into dict of SNPs passing various
    # criteria. Do this for each sample. Write to file.
    #==========================================================================
    snp_list_file_path = options_dict['snpListFile']
    vcf_file_name = options_dict['vcfFileName']
    list_of_vcf_files = [os.path.join(dir, vcf_file_name) for dir in list_of_sample_directories]

    bad_file_count = utils.verify_non_empty_input_files("VCF file", list_of_vcf_files)
    if bad_file_count == len(list_of_vcf_files):
        utils.global_error("Error: all %d VCF files were missing or empty." % bad_file_count)
    elif bad_file_count > 0:
        utils.sample_error("Error: %d VCF files were missing or empty." % bad_file_count, continue_possible=True)

    if options_dict['forceFlag'] or utils.target_needs_rebuild(list_of_vcf_files, snp_list_file_path):
        snp_dict = utils.convert_vcf_files_to_snp_dict(list_of_vcf_files)
        verbose_print('Found %d snp positions across %d sample vcf files.' % (len(snp_dict), len(list_of_vcf_files)))
        utils.write_list_of_snps(snp_list_file_path, snp_dict)
        verbose_print("")
    else:
        verbose_print("SNP list %s has already been freshly built.  Use the -f option to force a rebuild." % snp_list_file_path)
    verbose_print("# %s %s finished" % (utils.timestamp(), utils.program_name()))