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pyfastg: a minimal Python library for parsing SPAdes FASTG files

pyfastg CI Code Coverage PyPI bioconda

The FASTG file format

FASTG is a format for describing sequencing assembly graphs. It is geared toward accurately representing the ambiguity resulting from sequencing limitations, ploidy, or other factors that complicate representation of a seqence as a simple string.

The latest specification for the FASTG format is version 1.00, as of writing; the original FASTG website is down, but an archived version of the v1.00 specification is accessible here. Whenever the rest of this documentation mentions "the FASTG spec," this is in reference to this version of the specification.

pyfastg is a Python library designed to parse graphs that follow a subset of the FASTG spec. In particular, pyfastg is designed to work with files output by the SPAdes family of assemblers.

The pyfastg library

The pyfastg library contains parse_fastg(), a function that takes as input a path to a SPAdes FASTG file. parse_fastg() reads this FASTG file and returns a NetworkX DiGraph object representing the structure of the assembly graph.

pyfastg is useful as a starting point for other applications. Using this NetworkX DiGraph object, we can do whatever we want with the assembly graph: analyze it, convert it to other formats, visualize it, etc.

Note about the graph topology

The FASTG spec contains the following sentence (in section 6, page 7):

Note also that strictly speaking, [the structure described in a FASTG file] is not a graph at all, as we have not specified a notion of vertex. However in many cases one can without ambiguity define vertices and thereby associate a bona fide digraph, and we do so frequently in this document to illustrate concepts.

We use the following approach to get around this problem: "edges" in the FASTG file will be represented as nodes in the NetworkX graph produced by pyfastg, and "adjacencies" between edges in the FASTG file will be represented as edges in the NetworkX graph produced by pyfastg.

As far as we're aware, this "conversion" from edges to nodes matches how FASTG files have often been visualized in the past.

Installation

pyfastg can be installed using pip or conda:

Installation using pip

pip install pyfastg

Installation using conda

conda install -c bioconda pyfastg

Dependencies

As of writing, pyfastg's only direct dependency (which should be installed automatically when running either of the above installation commands) is NetworkX. pyfastg requires a minimum NetworkX version of 2.

As of writing, pyfastg supports Python 3.6 and up. pyfastg might be able to work with earlier versions of Python, but we do not explicitly test against these.

Quick example: using pyfastg to load and analyze an assembly graph

The second line (which points to one of pyfastg's test assembly graphs) assumes that you're located in the root directory of the pyfastg repo.

>>> import pyfastg
>>> g = pyfastg.parse_fastg("pyfastg/tests/input/assembly_graph.fastg")
>>> # g is now a NetworkX DiGraph! We can do whatever we want with this object.
>>>
>>> # Example: List the sequences in this graph (these are "edges" in the FASTG
>>> # file, but are represented as nodes in g)
>>> g.nodes()
NodeView(('1+', '29-', '1-', '6-', '2+', '26+', '27+', '2-', '3+', '4+', '6+', '7+', '3-', '33-', '9-', '4-', '5+', '5-', '28+', '7-', '8+', '28-', '9+', '8-', '12-', '10+', '12+', '10-', '24-', '32-', '11+', '30-', '11-', '27-', '19-', '13+', '25+', '31-', '13-', '14+', '14-', '26-', '15+', '15-', '23-', '16+', '16-', '17+', '17-', '19+', '18+', '33+', '18-', '20+', '20-', '22+', '21+', '21-', '22-', '23+', '24+', '25-', '29+', '30+', '31+', '32+'))
>>>
>>> # Example: Get details for a single sequence (length, coverage, GC-content)
>>> g.nodes["15+"]
{'length': 193, 'cov': 6.93966, 'gc': 0.5492227979274611}
>>>
>>> # Example: Get information about the graph's connectivity
>>> import networkx as nx
>>> components = list(nx.weakly_connected_components(g))
>>> for c in components:
...     print(len(c), "nodes")
...     print(c)
...
33 nodes
{'8-', '17-', '15+', '30+', '16+', '26-', '25+', '19+', '7+', '23+', '14-', '18-', '10-', '29-', '20-', '27-', '11-', '5-', '3+', '2-', '12-', '13+', '31-', '6+', '1+', '21-', '24-', '32-', '22+', '28+', '4+', '33-', '9-'}
33 nodes
{'26+', '29+', '18+', '3-', '2+', '8+', '15-', '24+', '9+', '17+', '27+', '28-', '11+', '6-', '20+', '14+', '19-', '13-', '4-', '21+', '5+', '31+', '22-', '12+', '25-', '30-', '10+', '1-', '7-', '32+', '23-', '33+', '16-'}

Details about the required input file format (tl;dr: SPAdes-dialect FASTG files only)

Currently, pyfastg is hardcoded to parse FASTG files created by the SPAdes assembler. Other valid FASTG files that don't follow the pattern used by SPAdes for edge names are not supported.

Edge names

Each edge in the file must have a name formatted like:

EDGE_1_length_9909_cov_6.94721

The edge ID (here, 1) can contain the characters a-z, A-Z, and 0-9.

The edge length (here, 9909) can contain the characters 0-9.

The edge coverage (here, 6.94721) can contain the characters 0-9 and ..

An edge name can optionally end with a ' character, indicating that this edge is a reverse complement. We will refer to whether or not an edge name ends with ' as its orientation: an edge that does not end with a ' has a + orientation, and an edge name that ends with a ' has a - orientation.

All edge names in a FASTG file should be consistent with respect to a given ID and orientation. If, in a single FASTG file, pyfastg sees a reference to an edge named EDGE_1_length_9909_cov_6.94721 and also a reference to an edge named EDGE_1_length_9908_cov_6.95 (with the same ID [1] and orientation [+], but a different length and/or coverage) then it will throw an error.

Edge declaration lines

Here, we refer to each line starting with > as an edge declaration. An edge's sequence is described in the line(s) following its edge declaration (until the next edge declaration); additionally, the outgoing adjacencies from this edge to other edges may be described on this line, if present. For example, the line

>EDGE_1_length_5_cov_10:EDGE_2_length_3_cov_1,EDGE_3_length_6_cov_2.5',EDGE_4_length_8_cov_5.1;

indicates that the edge EDGE_1_length_5_cov_10 has three outgoing adjacencies: to the edges EDGE_2_length_3_cov_1, EDGE_3_length_6_cov_2.5', and EDGE_4_length_8_cov_5.1. This line would thus result in three "edges" being created in the NetworkX graph produced by pyfastg: (1+2+), (1+3-), and (1+4+).

Each edge declaration must end with a ; character (after removing trailing whitespace). Section 15 of the FASTG spec mentions that having a newline after the semicolon isn't required, but we require it here for the sake of simplicity.

Edge sequences

We assume that each sequence (the line(s) between edge declarations) consists only of the characters A, C, G, T, or U. So, more complex types of strings (e.g. the "stuffed gaps" described in the FASTG spec) are not allowed in an edge's sequence.

Additionally, lowercase characters or degenerate nucleotides are not allowed; this matches section 15 of the FASTG spec. The FASTG spec doesn't explicitly allow for uracil (U), but we allow it anyway in order to support RNA sequences. (U and T are allowed to be contained in the same sequence, in the unlikely case that this is needed.)

Leading and trailing whitespace in sequence lines will be ignored, as will blank lines within a sequence. So, something like

>EDGE_1_length_4_cov_100;
    ATC

 G     

is technically valid: this sequence is read as ATCG. However, the following example:

>EDGE_1_length_4_cov_100;
ATC G

is not valid and will cause pyfastg to throw an error. This is because the inner space between the C and the G would be read as part of the sequence.

Details about the output NetworkX graph

Node names and attributes

Nodes in the returned DiGraph (corresponding to edges in the FASTG file) will contain three attribute fields:

  1. length: the length of the sequence (represented as a python int)
  2. cov: the coverage of the sequence (represented as a python float)
  3. gc: the GC-content (in the range [0, 1]) of the sequence (represented as a python float)

Each node's name is a python str created by concatenating edge IDs and orientations. For example, EDGE_1_length_9909_cov_6.94721 will correspond to a node named 1+. This naming scheme is analogous to that used by Bandage.

About reverse complements

pyfastg only creates nodes based on the edges explicitly described in the FASTG file. If a file only describes edges EDGE_1_length_5_cov_10, EDGE_2_length_6_cov_10', and EDGE_3_length_7_cov_15, then pyfastg will only create nodes 1+, 2-, and 3+, and not the reverse complement nodes 1-, 2+, 3-, etc.

Similarly, if a file contains an adjacency from edge EDGE_1_length_5_cov_10 to EDGE_2_length_6_cov_10', then this adjacency will only be represented as a single edge (1+2-) in pyfastg's output graph. The implied reverse-complement of this edge (2+1-) will not be created unless the file explicitly contains an adjacency from EDGE_2_length_6_cov_10 to EDGE_1_length_5_cov_10'.

Information for pyfastg developers

Pull requests are welcome! If you're interested in developing pyfastg's code, this section provides some instructions for getting started.

Setting up a development "environment" for pyfastg

You will probably want to fork this repository and then clone your fork to your computer. Once you do this, cd into the root of the repository and run

pip install -e .[dev]

to install pyfastg in "editable mode." Thanks to the [dev] flag, this will also install pyfastg's development dependencies (see the extras_require line in setup.py for details).

Testing, linting, and formatting the code

pyfastg's Makefile contains targets that perform these three tasks:

  • Run tests: make test
  • Lint and style-check the code: make stylecheck
  • Automatically style the code: make style

These targets should all be run from the root of the pyfastg repository. They should hopefully be self-explanatory, but let us know if you have any questions.

Changelog

See pyfastg's CHANGELOG.md file for information on the changes included with new pyfastg releases.

License

pyfastg is licensed under the MIT License. Please see pyfastg's LICENSE file for details.

Contact

The recommended way to get in touch with pyfastg's developers is by opening a GitHub issue.