Package Bio :: Package SeqIO :: Module QualityIO
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Module QualityIO

source code

Bio.SeqIO support for the "fastq" and "qual" file formats.

Note that you are expected to use this code via the Bio.SeqIO interface, as shown below.

The FASTQ file format is used frequently at the Wellcome Trust Sanger Institute to bundle a FASTA sequence and its PHRED quality data (integers between 0 and 90). Rather than using a single FASTQ file, often paired FASTA and QUAL files are used containing the sequence and the quality information separately.

The PHRED software reads DNA sequencing trace files, calls bases, and assigns a quality value between 0 and 90 to each called base using a logged transformation of the error probability, Q = -10 log10( Pe ), for example:

   Pe = 0.0,         Q =  0
   Pe = 0.1,         Q = 10
   Pe = 0.01,        Q = 20
   ...
   Pe = 0.00000001,  Q = 80
   Pe = 0.000000001, Q = 90

In the QUAL format these quality values are held as space separated text in a FASTA like file format. In the FASTQ format, each quality values is encoded with a single ASCI character using chr(Q+33), meaning zero maps to the character "!" and for example 80 maps to "q". The sequences and quality are then stored in pairs in a FASTA like format.

Unfortunately there is no official document describing the FASTQ file format, and worse, several related but different variants exist. Reasonable documentation exists at: http://maq.sourceforge.net/fastq.shtml

Solexa/Illumina quality scores use Q = - 10 log10 ( Pe / (1-Pe) ), which can be negative or easily exceed 90. PHRED scores and Solexa scores are NOT interchangeable (but a reasonable mapping can be achieved between them). Confusingly Solexa produces a FASTQ like file but using their own score mapping instead.

Also note that Roche 454 sequencers can output files in the QUAL format, and thankfully they use PHREP style scores like Sanger. To extract QUAL files from a Roche 454 SFF binary file, use the Roche off instrument command line tool "sffinfo" with the -q or -qual argument. You can extract a matching FASTA file using the -s or -seq argument instead.

You are expected to use this module via the Bio.SeqIO functions, with the following format names:

For example, consider the following short FASTQ file (extracted from a real NCBI dataset):

   @EAS54_6_R1_2_1_413_324
   CCCTTCTTGTCTTCAGCGTTTCTCC
   +
   ;;3;;;;;;;;;;;;7;;;;;;;88
   @EAS54_6_R1_2_1_540_792
   TTGGCAGGCCAAGGCCGATGGATCA
   +
   ;;;;;;;;;;;7;;;;;-;;;3;83
   @EAS54_6_R1_2_1_443_348
   GTTGCTTCTGGCGTGGGTGGGGGGG
   +
   ;;;;;;;;;;;9;7;;.7;393333

This contains three reads of length 25. From the read length these were probably originally from an early Solexa/Illumina sequencer but NCBI have followed the Sanger FASTQ convention and this actually uses PHRED style qualities. This means we can parse this file using Bio.SeqIO using "fastq" as the format name:

>>> from Bio import SeqIO
>>> for record in SeqIO.parse(open("Quality/example.fastq"), "fastq") :
...     print record.id, record.seq
EAS54_6_R1_2_1_413_324 CCCTTCTTGTCTTCAGCGTTTCTCC
EAS54_6_R1_2_1_540_792 TTGGCAGGCCAAGGCCGATGGATCA
EAS54_6_R1_2_1_443_348 GTTGCTTCTGGCGTGGGTGGGGGGG

The qualities are held as a list of integers in each record's annotation:

>>> print record
ID: EAS54_6_R1_2_1_443_348
Name: EAS54_6_R1_2_1_443_348
Description: EAS54_6_R1_2_1_443_348
Number of features: 0
Per letter annotation for: phred_quality
Seq('GTTGCTTCTGGCGTGGGTGGGGGGG', SingleLetterAlphabet())
>>> print record.letter_annotations["phred_quality"]
[26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 24, 26, 22, 26, 26, 13, 22, 26, 18, 24, 18, 18, 18, 18]

You can use the SeqRecord format method you can show this in the QUAL format:

>>> print record.format("qual")
>EAS54_6_R1_2_1_443_348
26 26 26 26 26 26 26 26 26 26 26 24 26 22 26 26 13 22 26 18
24 18 18 18 18
<BLANKLINE>

Or go back to the FASTQ format,

>>> print record.format("fastq")
@EAS54_6_R1_2_1_443_348
GTTGCTTCTGGCGTGGGTGGGGGGG
+
;;;;;;;;;;;9;7;;.7;393333
<BLANKLINE>

You can also get Biopython to convert the scores and show a Solexa style FASTQ file:

>>> print record.format("fastq-solexa")
@EAS54_6_R1_2_1_443_348
GTTGCTTCTGGCGTGGGTGGGGGGG
+
ZZZZZZZZZZZXZVZZMVZRXRRRR
<BLANKLINE>

If you wanted to trim your sequences (perhaps to remove low quality regions, or to remove a primer sequence), try slicing the SeqRecord objects. e.g.

>>> sub_rec = record[5:15]
>>> print sub_rec
ID: EAS54_6_R1_2_1_443_348
Name: EAS54_6_R1_2_1_443_348
Description: EAS54_6_R1_2_1_443_348
Number of features: 0
Per letter annotation for: phred_quality
Seq('TTCTGGCGTG', SingleLetterAlphabet())
>>> print sub_rec.letter_annotations["phred_quality"]
[26, 26, 26, 26, 26, 26, 24, 26, 22, 26]
>>> print sub_rec.format("fastq")
@EAS54_6_R1_2_1_443_348
TTCTGGCGTG
+
;;;;;;9;7;
<BLANKLINE>

If you wanted to, you could read in this FASTQ file, and save it as a QUAL file:

>>> from Bio import SeqIO
>>> record_iterator = SeqIO.parse(open("Quality/example.fastq"), "fastq")
>>> out_handle = open("Quality/temp.qual", "w")
>>> SeqIO.write(record_iterator, out_handle, "qual")
3
>>> out_handle.close()

You can of course read in a QUAL file, such as the one we just created:

>>> from Bio import SeqIO
>>> for record in SeqIO.parse(open("Quality/temp.qual"), "qual") :
...     print record.id, record.seq
EAS54_6_R1_2_1_413_324 ?????????????????????????
EAS54_6_R1_2_1_540_792 ?????????????????????????
EAS54_6_R1_2_1_443_348 ?????????????????????????

Notice that QUAL files don't have a proper sequence present! But the quality information is there:

>>> print record
ID: EAS54_6_R1_2_1_443_348
Name: EAS54_6_R1_2_1_443_348
Description: EAS54_6_R1_2_1_443_348
Number of features: 0
Per letter annotation for: phred_quality
UnknownSeq(25, alphabet = SingleLetterAlphabet(), character = '?')
>>> print record.letter_annotations["phred_quality"]
[26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 24, 26, 22, 26, 26, 13, 22, 26, 18, 24, 18, 18, 18, 18]

Just to keep things tidy, if you are following this example yourself, you can delete this temporary file now:

>>> import os
>>> os.remove("Quality/temp.qual")

Sometimes you won't have a FASTQ file, but rather just a pair of FASTA and QUAL files. Because the Bio.SeqIO system is designed for reading single files, you would have to read the two in separately and then combine the data. However, since this is such a common thing to want to do, there is a helper iterator defined in this module that does this for you - PairedFastaQualIterator.

Alternatively, if you have enough RAM to hold all the records in memory at once, then a simple dictionary approach would work:

>>> from Bio import SeqIO
>>> reads = SeqIO.to_dict(SeqIO.parse(open("Quality/example.fasta"), "fasta"))
>>> for rec in SeqIO.parse(open("Quality/example.qual"), "qual") :
...     reads[rec.id].letter_annotations["phred_quality"]=rec.letter_annotations["phred_quality"]

You can then access any record by its key, and get both the sequence and the quality scores.

>>> print reads["EAS54_6_R1_2_1_540_792"].format("fastq")
@EAS54_6_R1_2_1_540_792
TTGGCAGGCCAAGGCCGATGGATCA
+
;;;;;;;;;;;7;;;;;-;;;3;83
<BLANKLINE>

It is important that you explicitly tell Bio.SeqIO which FASTQ variant you are using ("fastq" for the Sanger standard using PHRED values, or "fastq-solexa" for the Solexa/Illumina variant), as this cannot be detected reliably automatically.

Classes [hide private]
  FastqPhredWriter
Class to write FASTQ format files (using PHRED quality scores).
  QualPhredWriter
Class to write QUAL format files (using PHRED quality scores).
  FastqSolexaWriter
Class to write FASTQ format files (using Solexa quality scores).
Functions [hide private]
 
solexa_quality_from_phred(phred_quality)
Covert a PHRED quality (range 0 to about 90) to a Solexa quality.
source code
 
phred_quality_from_solexa(solexa_quality)
Convert a Solexa quality (which can be negative) to a PHRED quality.
source code
 
_get_phred_quality(record)
Extract PHRED qualities from a SeqRecord's letter_annotations (PRIVATE).
source code
 
_get_solexa_quality(record)
Extract Solexa qualities from a SeqRecord's letter_annotations (PRIVATE).
source code
 
FastqGeneralIterator(handle)
Iterate over Fastq records as string tuples (not as SeqRecord objects).
source code
 
FastqPhredIterator(handle, alphabet=SingleLetterAlphabet(), title2ids=None)
Generator function to iterate over FASTQ records (as SeqRecord objects).
source code
 
FastqSolexaIterator(handle, alphabet=SingleLetterAlphabet(), title2ids=None)
Parsing the Solexa/Illumina FASTQ like files (which differ in the quality mapping).
source code
 
QualPhredIterator(handle, alphabet=SingleLetterAlphabet(), title2ids=None)
For QUAL files which include PHRED quality scores, but no sequence.
source code
 
PairedFastaQualIterator(fasta_handle, qual_handle, alphabet=SingleLetterAlphabet(), title2ids=None)
Iterate over matched FASTA and QUAL files as SeqRecord objects.
source code
 
_test()
Run the Bio.SeqIO module's doctests.
source code
Variables [hide private]
  SANGER_SCORE_OFFSET = 33
  SOLEXA_SCORE_OFFSET = 64
Function Details [hide private]

solexa_quality_from_phred(phred_quality)

source code 

Covert a PHRED quality (range 0 to about 90) to a Solexa quality.

This will return a floating point number, it is up to you to round this to the nearest integer if appropriate. e.g.

>>> print "%0.2f" % round(solexa_quality_from_phred(80),2)
80.00
>>> print "%0.2f" % round(solexa_quality_from_phred(50),2)
50.00
>>> print "%0.2f" % round(solexa_quality_from_phred(20),2)
19.96
>>> print "%0.2f" % round(solexa_quality_from_phred(10),2)
9.54
>>> print "%0.2f" % round(solexa_quality_from_phred(1),2)
-5.87

phred_quality_from_solexa(solexa_quality)

source code 

Convert a Solexa quality (which can be negative) to a PHRED quality.

This will return a floating point number, it is up to you to round this to the nearest integer if appropriate. e.g.

>>> print "%0.2f" % round(phred_quality_from_solexa(80),2)
80.00
>>> print "%0.2f" % round(phred_quality_from_solexa(20),2)
20.04
>>> print "%0.2f" % round(phred_quality_from_solexa(10),2)
10.41
>>> print "%0.2f" % round(phred_quality_from_solexa(0),2)
3.01
>>> print "%0.2f" % round(phred_quality_from_solexa(-10),2)
0.41

_get_phred_quality(record)

source code 

Extract PHRED qualities from a SeqRecord's letter_annotations (PRIVATE).

If there are no PHRED qualities, but there are Solexa qualities, those are used instead after conversion.

_get_solexa_quality(record)

source code 

Extract Solexa qualities from a SeqRecord's letter_annotations (PRIVATE).

If there are no Solexa qualities, but there are PHRED qualities, those are used instead after conversion.

FastqGeneralIterator(handle)

source code 

Iterate over Fastq records as string tuples (not as SeqRecord objects).

This code does not try to interpret the quality string numerically. It just returns tuples of the title, sequence and quality as strings. For the sequence and quality, any whitespace (such as new lines) is removed.

Our SeqRecord based FASTQ iterators call this function internally, and then turn the strings into a SeqRecord objects, mapping the quality string into a list of numerical scores. If you want to do a custom quality mapping, then you might consider calling this function directly.

For parsing FASTQ files, the title string from the "@" line at the start of each record can optionally be omitted on the "+" lines. If it is repeated, it must be identical.

The sequence string and the quality string can optionally be split over multiple lines, although several sources discourage this. In comparison, for the FASTA file format line breaks between 60 and 80 characters are the norm.

WARNING - Because the "@" character can appear in the quality string, this can cause problems as this is also the marker for the start of a new sequence. In fact, the "+" sign can also appear as well. Some sources recommended having no line breaks in the quality to avoid this, but even that is not enough, consider this example:

   @071113_EAS56_0053:1:1:998:236
   TTTCTTGCCCCCATAGACTGAGACCTTCCCTAAATA
   +071113_EAS56_0053:1:1:998:236
   IIIIIIIIIIIIIIIIIIIIIIIIIIIIICII+III
   @071113_EAS56_0053:1:1:182:712
   ACCCAGCTAATTTTTGTATTTTTGTTAGAGACAGTG
   +
   @IIIIIIIIIIIIIIICDIIIII<%<6&-*).(*%+
   @071113_EAS56_0053:1:1:153:10
   TGTTCTGAAGGAAGGTGTGCGTGCGTGTGTGTGTGT
   +
   IIIIIIIIIIIICIIGIIIII>IAIIIE65I=II:6
   @071113_EAS56_0053:1:3:990:501
   TGGGAGGTTTTATGTGGA
   AAGCAGCAATGTACAAGA
   +
   IIIIIII.IIIIII1@44
   @-7.%<&+/$/%4(++(%

This is four PHRED encoded FASTQ entries originally from an NCBI source (given the read length of 36, these are probably Solexa Illumna reads where the quality has been mapped onto the PHRED values).

This example has been edited to illustrate some of the nasty things allowed in the FASTQ format. Firstly, on the "+" lines most but not all of the (redundant) identifiers are ommited. In real files it is likely that all or none of these extra identifiers will be present.

Secondly, while the first three sequences have been shown without line breaks, the last has been split over multiple lines. In real files any line breaks are likely to be consistent.

Thirdly, some of the quality string lines start with an "@" character. For the second record this is unavoidable. However for the fourth sequence this only happens because its quality string is split over two lines. A naive parser could wrongly treat any line starting with an "@" as the beginning of a new sequence! This code copes with this possible ambiguity by keeping track of the length of the sequence which gives the expected length of the quality string.

Using this tricky example file as input, this short bit of code demonstrates what this parsing function would return:

>>> handle = open("Quality/tricky.fastq", "rU")
>>> for (title, sequence, quality) in FastqGeneralIterator(handle) :
...     print title
...     print sequence, quality
071113_EAS56_0053:1:1:998:236
TTTCTTGCCCCCATAGACTGAGACCTTCCCTAAATA IIIIIIIIIIIIIIIIIIIIIIIIIIIIICII+III
071113_EAS56_0053:1:1:182:712
ACCCAGCTAATTTTTGTATTTTTGTTAGAGACAGTG @IIIIIIIIIIIIIIICDIIIII<%<6&-*).(*%+
071113_EAS56_0053:1:1:153:10
TGTTCTGAAGGAAGGTGTGCGTGCGTGTGTGTGTGT IIIIIIIIIIIICIIGIIIII>IAIIIE65I=II:6
071113_EAS56_0053:1:3:990:501
TGGGAGGTTTTATGTGGAAAGCAGCAATGTACAAGA IIIIIII.IIIIII1@44@-7.%<&+/$/%4(++(%
>>> handle.close()

Finally we note that some sources state that the quality string should start with "!" (which using the PHRED mapping means the first letter always has a quality score of zero). This rather restrictive rule is not widely observed, so is therefore ignored here. One plus point about this "!" rule is that (provided there are no line breaks in the quality sequence) it would prevent the above problem with the "@" character.

FastqPhredIterator(handle, alphabet=SingleLetterAlphabet(), title2ids=None)

source code 

Generator function to iterate over FASTQ records (as SeqRecord objects).

  • handle - input file
  • alphabet - optional alphabet
  • title2ids - A function that, when given the title line from the FASTQ file (without the beginning >), will return the id, name and description (in that order) for the record as a tuple of strings. If this is not given, then the entire title line will be used as the description, and the first word as the id and name.

Note that use of title2ids matches that of Bio.SeqIO.FastaIO.

For each sequence in a (Sanger style) FASTQ file there is a matching string encoding the PHRED qualities (integers between 0 and about 90) using ASCII values with an offset of 33.

For example, consider a file containing three short reads:

   @EAS54_6_R1_2_1_413_324
   CCCTTCTTGTCTTCAGCGTTTCTCC
   +
   ;;3;;;;;;;;;;;;7;;;;;;;88
   @EAS54_6_R1_2_1_540_792
   TTGGCAGGCCAAGGCCGATGGATCA
   +
   ;;;;;;;;;;;7;;;;;-;;;3;83
   @EAS54_6_R1_2_1_443_348
   GTTGCTTCTGGCGTGGGTGGGGGGG
   +
   ;;;;;;;;;;;9;7;;.7;393333

For each sequence (e.g. "CCCTTCTTGTCTTCAGCGTTTCTCC") there is a matching string encoding the PHRED qualities using a ASCI values with an offset of 33 (e.g. ";;3;;;;;;;;;;;;7;;;;;;;88").

Using this module directly you might run:

>>> handle = open("Quality/example.fastq", "rU")
>>> for record in FastqPhredIterator(handle) :
...     print record.id, record.seq
EAS54_6_R1_2_1_413_324 CCCTTCTTGTCTTCAGCGTTTCTCC
EAS54_6_R1_2_1_540_792 TTGGCAGGCCAAGGCCGATGGATCA
EAS54_6_R1_2_1_443_348 GTTGCTTCTGGCGTGGGTGGGGGGG
>>> handle.close()

Typically however, you would call this via Bio.SeqIO instead with "fastq" as the format:

>>> from Bio import SeqIO
>>> handle = open("Quality/example.fastq", "rU")
>>> for record in SeqIO.parse(handle, "fastq") :
...     print record.id, record.seq
EAS54_6_R1_2_1_413_324 CCCTTCTTGTCTTCAGCGTTTCTCC
EAS54_6_R1_2_1_540_792 TTGGCAGGCCAAGGCCGATGGATCA
EAS54_6_R1_2_1_443_348 GTTGCTTCTGGCGTGGGTGGGGGGG
>>> handle.close()

If you want to look at the qualities, they are record in each record's per-letter-annotation dictionary as a simple list of integers:

>>> print record.letter_annotations["phred_quality"]
[26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 24, 26, 22, 26, 26, 13, 22, 26, 18, 24, 18, 18, 18, 18]

FastqSolexaIterator(handle, alphabet=SingleLetterAlphabet(), title2ids=None)

source code 

Parsing the Solexa/Illumina FASTQ like files (which differ in the quality mapping).

The optional arguments are the same as those for the FastqPhredIterator.

For each sequence in Solexa/Illumina FASTQ files there is a matching string encoding the Solexa integer qualities using ASCII values with an offset of 64. Solexa scores are scaled differently to PHRED scores, and Biopython will NOT perform any automatic conversion when loading.

For example, consider a file containing these five records:

   @SLXA-B3_649_FC8437_R1_1_1_610_79
   GATGTGCAATACCTTTGTAGAGGAA
   +SLXA-B3_649_FC8437_R1_1_1_610_79
   YYYYYYYYYYYYYYYYYYWYWYYSU
   @SLXA-B3_649_FC8437_R1_1_1_397_389
   GGTTTGAGAAAGAGAAATGAGATAA
   +SLXA-B3_649_FC8437_R1_1_1_397_389
   YYYYYYYYYWYYYYWWYYYWYWYWW
   @SLXA-B3_649_FC8437_R1_1_1_850_123
   GAGGGTGTTGATCATGATGATGGCG
   +SLXA-B3_649_FC8437_R1_1_1_850_123
   YYYYYYYYYYYYYWYYWYYSYYYSY
   @SLXA-B3_649_FC8437_R1_1_1_362_549
   GGAAACAAAGTTTTTCTCAACATAG
   +SLXA-B3_649_FC8437_R1_1_1_362_549
   YYYYYYYYYYYYYYYYYYWWWWYWY
   @SLXA-B3_649_FC8437_R1_1_1_183_714
   GTATTATTTAATGGCATACACTCAA
   +SLXA-B3_649_FC8437_R1_1_1_183_714
   YYYYYYYYYYWYYYYWYWWUWWWQQ

Using this module directly you might run:

>>> handle = open("Quality/solexa_example.fastq", "rU")
>>> for record in FastqSolexaIterator(handle) :
...     print record.id, record.seq
SLXA-B3_649_FC8437_R1_1_1_610_79 GATGTGCAATACCTTTGTAGAGGAA
SLXA-B3_649_FC8437_R1_1_1_397_389 GGTTTGAGAAAGAGAAATGAGATAA
SLXA-B3_649_FC8437_R1_1_1_850_123 GAGGGTGTTGATCATGATGATGGCG
SLXA-B3_649_FC8437_R1_1_1_362_549 GGAAACAAAGTTTTTCTCAACATAG
SLXA-B3_649_FC8437_R1_1_1_183_714 GTATTATTTAATGGCATACACTCAA
>>> handle.close()

Typically however, you would call this via Bio.SeqIO instead with "fastq" as the format:

>>> from Bio import SeqIO
>>> handle = open("Quality/solexa_example.fastq", "rU")
>>> for record in SeqIO.parse(handle, "fastq-solexa") :
...     print record.id, record.seq
SLXA-B3_649_FC8437_R1_1_1_610_79 GATGTGCAATACCTTTGTAGAGGAA
SLXA-B3_649_FC8437_R1_1_1_397_389 GGTTTGAGAAAGAGAAATGAGATAA
SLXA-B3_649_FC8437_R1_1_1_850_123 GAGGGTGTTGATCATGATGATGGCG
SLXA-B3_649_FC8437_R1_1_1_362_549 GGAAACAAAGTTTTTCTCAACATAG
SLXA-B3_649_FC8437_R1_1_1_183_714 GTATTATTTAATGGCATACACTCAA
>>> handle.close()

If you want to look at the qualities, they are recorded in each record's per-letter-annotation dictionary as a simple list of integers:

>>> print record.letter_annotations["solexa_quality"]
[25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 23, 25, 25, 25, 25, 23, 25, 23, 23, 21, 23, 23, 23, 17, 17]

These scores aren't very good, but they are high enough that they map almost exactly onto PHRED scores:

>>> print "%0.2f" % phred_quality_from_solexa(25)
25.01

Let's look at another example read which is even worse, where there are more noticeable differences between the Solexa and PHRED scores:

    @slxa_0013_1_0001_24
    ACAAAAATCACAAGCATTCTTATACACC
    +slxa_0013_1_0001_24
    ??????????????????:??<?<-6%.

Again, you would typically use Bio.SeqIO to read this file in (rather than calling the Bio.SeqIO.QualtityIO module directly). Most FASTQ files will contain thousands of reads, so you would normally use Bio.SeqIO.parse() as shown above. This example has only as one entry, so instead we can use the Bio.SeqIO.read() function:

>>> from Bio import SeqIO
>>> handle = open("Quality/solexa.fastq", "rU")
>>> record = SeqIO.read(handle, "fastq-solexa")
>>> handle.close()
>>> print record.id, record.seq
slxa_0013_1_0001_24 ACAAAAATCACAAGCATTCTTATACACC
>>> print record.letter_annotations["solexa_quality"]
[-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -6, -1, -1, -4, -1, -4, -19, -10, -27, -18]

These quality scores are so low that when converted from the Solexa scheme into PHRED scores they look quite different:

>>> print "%0.2f" % phred_quality_from_solexa(-1)
2.54

Note you can use the Bio.SeqIO.write() function or the SeqRecord's format method to output the record(s):

>>> print record.format("fastq-solexa")
@slxa_0013_1_0001_24
ACAAAAATCACAAGCATTCTTATACACC
+
??????????????????:??<?<-6%.
<BLANKLINE>

Note this output is slightly different from the input file as Biopython has left out the optional repetition of the sequence identifier on the "+" line. If you want the to use PHRED scores, use "fastq" or "qual" as the output format instead, and Biopython will do the conversion for you:

>>> print record.format("fastq")
@slxa_0013_1_0001_24
ACAAAAATCACAAGCATTCTTATACACC
+
$$$$$$$$$$$$$$$$$$"$$"$"!!!!
<BLANKLINE>
>>> print record.format("qual")
>slxa_0013_1_0001_24
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 1 3 3 1 3 1 0 0 0 0
<BLANKLINE>

QualPhredIterator(handle, alphabet=SingleLetterAlphabet(), title2ids=None)

source code 

For QUAL files which include PHRED quality scores, but no sequence.

For example, consider this short QUAL file:

   >EAS54_6_R1_2_1_413_324
   26 26 18 26 26 26 26 26 26 26 26 26 26 26 26 22 26 26 26 26
   26 26 26 23 23
   >EAS54_6_R1_2_1_540_792
   26 26 26 26 26 26 26 26 26 26 26 22 26 26 26 26 26 12 26 26
   26 18 26 23 18
   >EAS54_6_R1_2_1_443_348
   26 26 26 26 26 26 26 26 26 26 26 24 26 22 26 26 13 22 26 18
   24 18 18 18 18

Using this module directly you might run:

>>> handle = open("Quality/example.qual", "rU")
>>> for record in QualPhredIterator(handle) :
...     print record.id, record.seq
EAS54_6_R1_2_1_413_324 ?????????????????????????
EAS54_6_R1_2_1_540_792 ?????????????????????????
EAS54_6_R1_2_1_443_348 ?????????????????????????
>>> handle.close()

Typically however, you would call this via Bio.SeqIO instead with "qual" as the format:

>>> from Bio import SeqIO
>>> handle = open("Quality/example.qual", "rU")
>>> for record in SeqIO.parse(handle, "qual") :
...     print record.id, record.seq
EAS54_6_R1_2_1_413_324 ?????????????????????????
EAS54_6_R1_2_1_540_792 ?????????????????????????
EAS54_6_R1_2_1_443_348 ?????????????????????????
>>> handle.close()

Becase QUAL files don't contain the sequence string itself, the seq property is set to an UnknownSeq object. As no alphabet was given, this has defaulted to a generic single letter alphabet and the character "?" used.

By specifying a nucleotide alphabet, "N" is used instead:

>>> from Bio import SeqIO
>>> from Bio.Alphabet import generic_dna
>>> handle = open("Quality/example.qual", "rU")
>>> for record in SeqIO.parse(handle, "qual", alphabet=generic_dna) :
...     print record.id, record.seq
EAS54_6_R1_2_1_413_324 NNNNNNNNNNNNNNNNNNNNNNNNN
EAS54_6_R1_2_1_540_792 NNNNNNNNNNNNNNNNNNNNNNNNN
EAS54_6_R1_2_1_443_348 NNNNNNNNNNNNNNNNNNNNNNNNN
>>> handle.close()

However, the quality scores themselves are available as a list of integers in each record's per-letter-annotation:

>>> print record.letter_annotations["phred_quality"]
[26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 24, 26, 22, 26, 26, 13, 22, 26, 18, 24, 18, 18, 18, 18]

You can still slice one of these SeqRecord objects with an UnknownSeq:

>>> sub_record = record[5:10]
>>> print sub_record.id, sub_record.letter_annotations["phred_quality"]
EAS54_6_R1_2_1_443_348 [26, 26, 26, 26, 26]

PairedFastaQualIterator(fasta_handle, qual_handle, alphabet=SingleLetterAlphabet(), title2ids=None)

source code 

Iterate over matched FASTA and QUAL files as SeqRecord objects.

For example, consider this short QUAL file:

   >EAS54_6_R1_2_1_413_324
   26 26 18 26 26 26 26 26 26 26 26 26 26 26 26 22 26 26 26 26
   26 26 26 23 23
   >EAS54_6_R1_2_1_540_792
   26 26 26 26 26 26 26 26 26 26 26 22 26 26 26 26 26 12 26 26
   26 18 26 23 18
   >EAS54_6_R1_2_1_443_348
   26 26 26 26 26 26 26 26 26 26 26 24 26 22 26 26 13 22 26 18
   24 18 18 18 18

And a matching FASTA file:

   >EAS54_6_R1_2_1_413_324
   CCCTTCTTGTCTTCAGCGTTTCTCC
   >EAS54_6_R1_2_1_540_792
   TTGGCAGGCCAAGGCCGATGGATCA
   >EAS54_6_R1_2_1_443_348
   GTTGCTTCTGGCGTGGGTGGGGGGG

You can parse these separately using Bio.SeqIO with the "qual" and "fasta" formats, but then you'll get a group of SeqRecord objects with no sequence, and a matching group with the sequence but not the qualities. Because it only deals with one input file handle, Bio.SeqIO can't be used to read the two files together - but this function can! For example,

>>> rec_iter = PairedFastaQualIterator(open("Quality/example.fasta", "rU"),
...                                    open("Quality/example.qual", "rU"))
>>> for record in rec_iter :
...     print record.id, record.seq
EAS54_6_R1_2_1_413_324 CCCTTCTTGTCTTCAGCGTTTCTCC
EAS54_6_R1_2_1_540_792 TTGGCAGGCCAAGGCCGATGGATCA
EAS54_6_R1_2_1_443_348 GTTGCTTCTGGCGTGGGTGGGGGGG

As with the FASTQ or QUAL parsers, if you want to look at the qualities, they are in each record's per-letter-annotation dictionary as a simple list of integers:

>>> print record.letter_annotations["phred_quality"]
[26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 24, 26, 22, 26, 26, 13, 22, 26, 18, 24, 18, 18, 18, 18]

If you have access to data as a FASTQ format file, using that directly would be simpler and more straight forward. Note that you can easily use this function to convert paired FASTA and QUAL files into FASTQ files:

>>> from Bio import SeqIO
>>> rec_iter = PairedFastaQualIterator(open("Quality/example.fasta", "rU"),
...                                    open("Quality/example.qual", "rU"))
>>> out_handle = open("Quality/temp.fastq", "w")
>>> SeqIO.write(rec_iter, out_handle, "fastq")
3
>>> out_handle.close()

And don't forget to clean up the temp file if you don't need it anymore:

>>> import os
>>> os.remove("Quality/temp.fastq")

_test()

source code 

Run the Bio.SeqIO module's doctests.

This will try and locate the unit tests directory, and run the doctests from there in order that the relative paths used in the examples work.