# Writing PB in file

Table of Contents

- Fasta files
- Dihedral angles
- Read fasta files

Note

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The API allows to write all the files the command line tools can. This includes the outputs of PBassign. The functions to handle several file formats are available in the `pbxplore.io` module.

    from __future__ import print_function, division
    from pprint import pprint
    import os

    import pbxplore as pbx

    ---------------------------------------------------------------------------

    ImportError                               Traceback (most recent call last)

    <ipython-input-2-853cbdb98b68> in <module>()
    ----> 1 import pbxplore as pbx

    ImportError: No module named pbxplore

## Fasta files

The most common way to save PB sequences is to write them in a fasta file.

PBxplore allows two ways to write fasta files. The sequences can be written either all at once or one at a time. To write a batch of sequences at once, we need a list of sequences and a list of the corresponding sequence names. The writing function here is `pbxplore.io.write_fasta()`.

    names = []
    pb_sequences = []
    pdb_path = os.path.join(pbx.DEMO_DATA_PATH, '2LFU.pdb')
    for chain_name, chain in pbx.chains_from_files([pdb_path]):
        dihedrals = chain.get_phi_psi_angles()
        pb_seq = pbx.assign(dihedrals)
        names.append(chain_name)
        pb_sequences.append(pb_seq)

    pprint(names)

    pprint(pb_sequences)

    with open('output.fasta', 'w') as outfile:
        pbx.io.write_fasta(outfile, pb_sequences, names)

    ---------------------------------------------------------------------------

    NameError                                 Traceback (most recent call last)

    <ipython-input-3-feec9831b75c> in <module>()
          1 names = []
          2 pb_sequences = []
    ----> 3 pdb_path = os.path.join(pbx.DEMO_DATA_PATH, '2LFU.pdb')
          4 for chain_name, chain in pbx.chains_from_files([pdb_path]):
          5     dihedrals = chain.get_phi_psi_angles()

    NameError: name 'pbx' is not defined

    !cat output.fasta
    !rm output.fasta

    cat: output.fasta: Aucun fichier ou dossier de ce type
    rm: impossible de supprimer 'output.fasta': Aucun fichier ou dossier de ce type

Sequences can be written once at a time using the `pbxplore.io.write_fasta_entry()` function.

    pdb_path = os.path.join(pbx.DEMO_DATA_PATH, '2LFU.pdb')
    with open('output.fasta', 'w') as outfile:
        for chain_name, chain in pbx.chains_from_files([pdb_path]):
            dihedrals = chain.get_phi_psi_angles()
            pb_seq = pbx.assign(dihedrals)
            pbx.io.write_fasta_entry(outfile, pb_seq, chain_name)

    ---------------------------------------------------------------------------

    NameError                                 Traceback (most recent call last)

    <ipython-input-5-ca18ad829ea9> in <module>()
    ----> 1 pdb_path = os.path.join(pbx.DEMO_DATA_PATH, '2LFU.pdb')
          2 with open('output.fasta', 'w') as outfile:
          3     for chain_name, chain in pbx.chains_from_files([pdb_path]):
          4         dihedrals = chain.get_phi_psi_angles()
          5         pb_seq = pbx.assign(dihedrals)

    NameError: name 'pbx' is not defined

    !cat output.fasta
    !rm output.fasta

    cat: output.fasta: Aucun fichier ou dossier de ce type
    rm: impossible de supprimer 'output.fasta': Aucun fichier ou dossier de ce type

By default, the lines in fasta files are wrapped at 60 caracters as defined in `pbxplore.io.fasta.FASTA_WIDTH`. Both `pbxplore.io.write_fasta()` and `pbxplore.io.write_fasta_entry()` have a `width` optionnal argument that allows to control the wrapping.

    print(pb_sequences[0])

    ---------------------------------------------------------------------------

    IndexError                                Traceback (most recent call last)

    <ipython-input-7-37179c94b1b9> in <module>()
    ----> 1 print(pb_sequences[0])

    IndexError: list index out of range

    with open('output.fasta', 'w') as outfile:
        for width in (60, 70, 80):
            pbx.io.write_fasta_entry(outfile, pb_sequences[0],
                                            'width={} blocks'.format(width),
                                            width=width)

    ---------------------------------------------------------------------------

    NameError                                 Traceback (most recent call last)

    <ipython-input-8-975b3b2c17a0> in <module>()
          1 with open('output.fasta', 'w') as outfile:
          2     for width in (60, 70, 80):
    ----> 3         pbx.io.write_fasta_entry(outfile, pb_sequences[0],
          4                                         'width={} blocks'.format(width),
          5                                         width=width)

    NameError: name 'pbx' is not defined

    !cat output.fasta
    !rm output.fasta

## Dihedral angles

One needs the phi and psi dihedral angles to assign protein block sequences. Having these angles, it is sometime convenient to store them in a file. This can be done easily.

    pdb_path = os.path.join(pbx.DEMO_DATA_PATH, '2LFU.pdb')

    with open('output.phipsi', 'w') as outfile:
        for chain_name, chain in pbx.chains_from_files([pdb_path]):
            dihedral = chain.get_phi_psi_angles()
            for res in sorted(dihedral):
                phi = "{:8.2f}".format(dihedral[res]["phi"]) if dihedral[res]["phi"] else "    None"
                psi = "{:8.2f}".format(dihedral[res]["psi"]) if dihedral[res]["psi"] else "    None"
                print("{} {:6d} {} {} ".format(chain_name, res, phi, psi), file=outfile)

    ---------------------------------------------------------------------------

    NameError                                 Traceback (most recent call last)

    <ipython-input-10-667e115945df> in <module>()
    ----> 1 pdb_path = os.path.join(pbx.DEMO_DATA_PATH, '2LFU.pdb')
          2
          3 with open('output.phipsi', 'w') as outfile:
          4     for chain_name, chain in pbx.chains_from_files([pdb_path]):
          5         dihedral = chain.get_phi_psi_angles()

    NameError: name 'pbx' is not defined

Note it’s better to write the dihedral for each PDB/frame due to the high memory cost to store all of them in a list.

The output is formated with one line per residue. The first columns repeat the name given for the chain, then is the residue id followed by the phi and the psi angle. If an angle is not defined, ‘None’ is written instead.

    !head output.phipsi
    !tail output.phipsi
    !rm output.phipsi

    head: impossible d'ouvrir 'output.phipsi' en lecture: Aucun fichier ou dossier de ce type
    tail: impossible d'ouvrir 'output.phipsi' en lecture: Aucun fichier ou dossier de ce type
    rm: impossible de supprimer 'output.phipsi': Aucun fichier ou dossier de ce type

## Read fasta files

We want to read sequences that we wrote in files. PBxplore provides a function to read fasta files: the `pbxplore.io.read_fasta()` function.

    def pdb_to_fasta_pb(pdb_path, fasta_path):
        """
        Write a fasta file with all the PB sequences from a PDB
        """
        with open(fasta_path, 'w') as outfile:
            for chain_name, chain in pbx.chains_from_files([pdb_path]):
                dihedrals = chain.get_phi_psi_angles()
                pb_seq = pbx.assign(dihedrals)
                pbx.io.write_fasta_entry(outfile, pb_seq, chain_name)

    # Write a fasta file
    pdb_path = os.path.join(pbx.DEMO_DATA_PATH, '2LFU.pdb')
    pdb_to_fasta_pb(pdb_path, 'output.fasta')

    # Read a list of headers and a list of sequences from a fasta file
    names, sequences = pbx.io.read_fasta('output.fasta')

    print('names:')
    pprint(names)
    print('sequences:')
    pprint(sequences)

    !rm output.fasta

    ---------------------------------------------------------------------------

    NameError                                 Traceback (most recent call last)

    <ipython-input-12-83ffe81d42a5> in <module>()
         10
         11 # Write a fasta file
    ---> 12 pdb_path = os.path.join(pbx.DEMO_DATA_PATH, '2LFU.pdb')
         13 pdb_to_fasta_pb(pdb_path, 'output.fasta')
         14

    NameError: name 'pbx' is not defined

If the sequences we want to read are spread amongst several fasta files, then we can use the `pbxplore.io.read_several_fasta()` function that takes a list of fasta file path as argument instead of a single path.

    # Write several fasta files
    pdb_to_fasta_pb(os.path.join(pbx.DEMO_DATA_PATH, '1BTA.pdb'), '1BTA.fasta')
    pdb_to_fasta_pb(os.path.join(pbx.DEMO_DATA_PATH, '2LFU.pdb'), '2FLU.fasta')

    # Read the fasta files
    names, sequences = pbx.io.read_several_fasta(['1BTA.fasta', '2FLU.fasta'])

    # Print the first entries
    print('names:')
    pprint(names[:5])
    print('sequences:')
    pprint(sequences[:5])

    !rm 1BTA.fasta 2FLU.fasta

    ---------------------------------------------------------------------------

    NameError                                 Traceback (most recent call last)

    <ipython-input-13-fe5b22c077f3> in <module>()
          1 # Write several fasta files
    ----> 2 pdb_to_fasta_pb(os.path.join(pbx.DEMO_DATA_PATH, '1BTA.pdb'), '1BTA.fasta')
          3 pdb_to_fasta_pb(os.path.join(pbx.DEMO_DATA_PATH, '2LFU.pdb'), '2FLU.fasta')
          4
          5 # Read the fasta files

    NameError: name 'pbx' is not defined