User Manual


Some doctests are included in screed examples. The examples in this document are meant for human consumption only. They will not work in doctests!

screed parses FASTA and FASTQ files, generates databases, and lets you query these databases. Values such as sequence name, sequence description, sequence quality, and the sequence itself can be retrieved from these databases.


The following software packages are required to run screed:

  • Python 2 (2.7) or Python 3 (3.3 or newer)
  • pytest (only required to running tests)

Use pip to download, and install Screed and its dependencies:

pip install screed

To run the optional tests type:

python -m screed.tests

Command-line Quick Start

Creating a database

$ screed db <fasta/fastq file>

Dumping a database to a file

$ screed dump_fasta <screed database file> <fasta output>
$ screed dump_fastq <screed database file> <fastq output>

If no output file is provided, sequences are written to the terminal (stdout) by default.

Python Quick Start

Reading FASTA/FASTQ files

>>> import screed
>>> with as seqfile:
>>>     for read in seqfile:
...         print(, read.sequence)

Here, filename can be a FASTA or FASTQ file, and can be uncompressed, gzip-compressed, or bzip2-compressed. screed natively supports FASTA and FASTQ databases creation. If your sequences are in a different format see the developer documentation on Writing Custom Sequence Parsers.

Creating a database

>>> import screed
>>> screed.make_db('screed/tests/test-data/test.fa')

This loads a FASTA file screed/tests/test-data/test.fa into a screed database named screed/tests/test-data/test.fa_screed. A couple of things to note:

  • The screed database is independent of the text file from which it was derived, so moving, renaming or deleting screed/tests/test-data/test.fa will not affect the newly created database.

  • The make_db function inferred the file type as FASTA automatically. The read_fasta_sequences() and read_fastq_sequences() functions are available if you’d prefer to be explicit.

    >>> screed.read_fasta_sequences('screed/tests/test-data/test.fasta')
    >>> screed.read_fastq_sequences('screed/tests/test-data/test.fastq')

Opening a database

The class ScreedDB is used to read screed databases, regardless of what file format they were derived from (FASTA/FASTQ/hava/etc.). One reader to rule them all!

From the Python prompt, import the ScreedDB class and load some databases:

>>> from screed import ScreedDB
>>> fadb = ScreedDB('screed/tests/test-data/test.fa')
>>> fqdb = ScreedDB('screed/tests/test-data/test.fastq')

Notice how you didn’t need to write the ‘_screed’ at the end of the file names? screed automatically adds that to the file name if you didn’t.

Database dictionary interface

Since screed emulates a read-only dictionary interface, any methods that don’t modify a dictionary are supported:

>>> fadb.keys()
>>> fqdb.keys()

Each record in the database contains ‘fields’ such as name and sequence information. If the database was derived from a FASTQ file, quality and optional annotation strings are included. Conversely, FASTA-derived databases have a description field.

To retrieve the names of records in the database:

>>> names = fadb.keys()

The size of the databases (number of sequence records) is easily found:

>>> len(fadb)
>>> len(fqdb)

Retrieving records from a database

A record is the standard container unit in screed. Each has fields that vary slightly depending on what kind of file the database was derived from. For instance, a FASTQ-derived screed database has an id, a name, a quality score and a sequence. A FASTA-derived screed database has an id, name, description and a sequence.

Retrieving entire records:

>>> records = [r for r in fadb.itervalues()]

Each record is a dictionary of fields. The names of fields are keys into this dictionary with the actual information as values. For example:

>>> record = fadb[fadb.keys()[0]]
>>> index = record['id']
>>> name = record['name']
>>> description = record['description']
>>> sequence = record['sequence']

What this does is retrieve the first record object in the screed database, then retrieve the index, name, description and sequence from the record object using standard dictionary key -> value pairs.

Retrieving partial sequences (slicing)

screed supports the concept of retrieving a slice or a subset of a sequence string. The motivation is speed: if you have a database entry with a very long sequence string but only want a small portion of the string, it is faster to retrieve only the portion than to retrieve the entire string and then perform standard Python string slicing.

By default, screed’s FASTA database creator sets up the sequence column to support slicing. For example, if you have an entry with name someSeq which has a 10K long sequence, and you want a slice of the sequence spanning positions 4000 to 4080:

>>> seq = db['someSeq'].sequence
>>> slice = seq[4000:4080]

This is much faster than say:

>>> seq = str(db['someSeq'].sequence)
>>> slice = seq[4000:4080]

Because deep down, less information is being read off the disk. The :code`str()` method above causes the entire sequence to be retrieved as a string. Then Python slicing is done on the string seq and the subset stored in slice.

Retrieving records via index

Sometimes you don’t care what the name of a sequence is; you’re only interested in its position in the database. In these cases, retrieval via index is the method you’ll want to use:

>>> record = fqdb.loadRecordByIndex(5)

An index is like an offset into the database. The order records were kept in the FASTA or FASTQ file determines the index in their resulting screed database. The first record in a sequence file will have an index of 0, the second, an index of 1 and so on.

File Formats As Understood By Screed

While the screed database remains non-specific to file formats, the included FASTA and FASTQ parsers expect specific formats. These parsers attempt to handle the most common attributes of sequence files, though they can not support all features.


The FASTQ parsing function is read_fastq_sequences() and is located in the screed module.

The first line in a record must begin with ‘@’ and is followed by a record identifier (a name). An optional annotations string may be included after a space on the same line.

The second line begins the sequence line(s) which may be line wrapped. screed defines no limit on the length of sequence lines and no length on how many sequence lines a record may contain.

After the sequence line(s) comes a ‘+’ character on a new line. Some FASTQ formats require the first line to be repeated after the ‘+’ character, but since this adds no new information to the record, read_fastq_sequences() will ignore this if it is included.

The quality line(s) is last. Like the sequence line(s) this may be line wrapped. read_fastq_sequences() will raise an exception if the quality and sequence strings are of unequal length. screed performs no checking for valid quality scores.


The FASTA parsing function is read_fasta_sequences() and is also located in the screed module.

The first line in a record must begin with ‘>’ and is followed with the sequence’s name and an optional description. If the description is included, it is separated from the name with a space. Note that though the FASTA format doesn’t require named records, screed does. Without a unique name, screed can’t look up sequences by name.

The second line begins the line(s) of sequence. Like the FASTQ parser, read_fasta_sequences() allows any number of lines of any length.

FASTA <-> FASTQ Conversion

As an extra nicety, screed can convert FASTA files to FASTQ and back again.


The function used for this process is called ‘ToFastq’ and is located in the screed module. It takes the path to a screed database as the first argument and a path to the desired FASTQ file as the second argument. There is also a shell interface if the screed module is in your PYTHONPATH:

$ python -m screed dump_fastq <path to fasta db> [ <converted fastq file> ]

The FASTA name attribute is directly dumped from the file. The sequence attribute is also dumped pretty much directly, but is line wrapped to 80 characters if it is longer.

Any description line in the FASTA database is stored as a FASTQ annotation string with no other interpretation done.

Finally, as there is no quality or quality score in a FASTA file, a default one is generated. The generation of the quality follows the Sanger FASTQ conventions. The score is 1 (ASCII: ‘”’) meaning a probability of about 75% that the read is incorrect (1 in 4 chance). This PHRED quality score is calculated from the Sanger format: Q = -10log(p) where p is the probability of an incorrect read. Obviously this is a very rough way of providing a quality score and it is only intended to fill in the requirements of a FASTQ file. Any application needing a true measurement of the quality should not rely on this automatic conversion.


The function used for this process is called ‘toFasta’ and is located in the screed module. It takes the path to a screed database as the first argument and a path to the desired FASTA file as the second argument. Like the ToFastq function before, there is a shell interface to ToFasta if the screed module is in your PYTHONPATH:

$ python -m screed dump_fasta <path to fastq db> [ <converted fasta file> ]

As above, the name and sequence attributes are directly dumped from the FASTQ database to the FASTA file with the sequence line wrapping to 80 characters.

If it exists, the FASTQ annotation tag is stored as the FASTA description tag. As there is no equivalent in FASTA, the FASTQ quality score is ignored.