Seq objects

Version Compatibility

Reference examples tested with: BioPython 1.83+

Before using code patterns, verify installed versions match. If versions differ:

• Python: pip show <package> then help(module.function) to check signatures

If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.

Seq Objects

Create and manipulate biological sequence objects using Biopython.

"Create a sequence object" -> Wrap a raw string in a typed sequence container for biological operations.

• Immutable: Seq('ATGC') (BioPython) - string-like, supports complement/translate
• Mutable: MutableSeq('ATGC') (BioPython) - supports in-place edits
• Annotated: SeqRecord(Seq(...), id=...) (BioPython) - adds metadata for file I/O

The governing principle: no alphabet, no validation

Bio.Alphabet was removed entirely in Biopython 1.78 (2020-09-04). Seq and SeqRecord lost their .alphabet attribute, and any old-style construction fails LOUD: from Bio.Alphabet import IUPAC raises ImportError, and Seq('ACGT', IUPAC.unambiguous_dna) raises TypeError. Molecule type now lives as a SeqRecord annotation, record.annotations['molecule_type'] = 'DNA', consumed by the GenBank/EMBL writers.

The consequence governs everything downstream: no Seq operation validates its alphabet anymore. A protein passed to reverse_complement() or transcribe() returns silent garbage rather than an error. Sibling skills (transcription-translation, reverse-complement) inherit this - the burden is on the caller to track what kind of molecule a Seq holds.

Required Imports

from Bio.Seq import Seq, MutableSeq
from Bio.SeqRecord import SeqRecord

Core Objects

Seq - Immutable Sequence

Immutable and behaves like str since 1.78: indexing, slicing, +, *, .upper(), in, .count(), .find() all work. In-place edits raise: seq[0] = 'A' -> TypeError (LOUD). Use MutableSeq for edits.

seq = Seq('ATGCGATCGATCG')

len(seq)           # length
seq[0]             # first base
seq[0:10]          # slice (returns Seq)
str(seq)           # text form (see bytes note below)
'ATG' in seq       # membership test
seq.count('G')     # count occurrences
seq.find('ATG')    # position (-1 if not found)
seq.upper()        # uppercase (returns Seq)
seq * 3            # repeat

Since 1.79 Seq is backed by bytes (and MutableSeq by bytearray), NOT a str subclass. Use str(seq) for text and bytes(seq) for bytes. isinstance(seq, str) is always False - code that type-checks with isinstance(x, str) to detect sequences silently skips every Seq; test isinstance(x, (Seq, MutableSeq)) instead.

MutableSeq - Mutable Sequence

A bytearray-backed sequence for in-place editing; required when an operation needs inplace=True.

mut_seq = MutableSeq('ATGCGATCG')
mut_seq[0] = 'C'              # modify single position
mut_seq[0:3] = 'GGG'          # replace slice
mut_seq.append('A')           # add to end
mut_seq.insert(0, 'G')        # insert at position
mut_seq.pop()                 # remove and return last
mut_seq.remove('G')           # remove first occurrence
mut_seq.reverse()             # reverse in place

Convert between types (a MutableSeq is unhashable and cannot be a dict key or used in SeqIO.write, so cast back to Seq when done editing):

seq = Seq(mut_seq)            # MutableSeq -> Seq
mut_seq = MutableSeq(seq)     # Seq -> MutableSeq

Undefined and partially-defined sequences

UndefinedSequenceError (added 1.79, a subclass of ValueError) models a sequence whose length is known but whose content is not - produced by lazy/partial file parsers. A Seq(None, length=20) reports len() == 20 but raises on any attempt to read the bytes.

undef = Seq(None, length=20)
len(undef)            # 20 - fine
str(undef)            # raises UndefinedSequenceError (subclass of ValueError)

partial = Seq({3: 'ACGT'}, length=10)   # only positions 3-6 defined
str(partial[3:7])     # 'ACGT' - defined region reads fine
str(partial)          # raises - undefined positions

Note: complement()/reverse_complement() on an undefined Seq return self rather than crash, but any read of the bytes raises. Guard reads of records from lazy parsers with try/except UndefinedSequenceError only where content access is genuinely optional.

SeqRecord - Annotated Sequence

Sequence plus metadata for file I/O and analysis.

record = SeqRecord(Seq('ATGCGATCG'), id='gene1', name='example_gene', description='An example gene sequence')

record.seq                 # the Seq object
record.id                  # identifier string
record.name                # name string
record.description         # description string
record.features            # list of SeqFeature objects
record.annotations         # dict (organism, molecule_type, topology, ...)
record.letter_annotations  # per-letter annotations (e.g. phred_quality)
record.dbxrefs             # database cross-references

SeqRecord transformations

Goal: Transform whole records (reverse-complement, translate, slice) while keeping metadata coherent.

Approach: Use SeqRecord methods that return new records with features remapped to the new coordinate frame; pass id/description explicitly because they are NOT carried automatically.

rc_record = record.reverse_complement(id=f'{record.id}_rc', description='reverse complement')
protein_record = record.translate(id=f'{record.id}_protein', to_stop=True)
fasta_str = record.format('fasta')      # quick in-memory file-format string

Slicing a SeqRecord remaps features but silently DROPS annotations, dbxrefs, and any feature that straddles a slice boundary - subset = record[10:50] returns a record with empty annotations. Re-attach molecule_type (and anything else a writer needs) on the slice before writing.

subset = record[10:50]                          # features clipped; annotations dropped
subset.annotations['molecule_type'] = 'DNA'     # restore before GenBank/EMBL write

Code Patterns

Create Seq from String

dna = Seq('ATGCGATCGATCG')
rna = Seq('AUGCGAUCGAUCG')
protein = Seq('MRCRS')

Create SeqRecord with annotations for GenBank output

record = SeqRecord(Seq('ATGCGATCG'), id='gene1', description='Example')
record.annotations['organism'] = 'Homo sapiens'
record.annotations['molecule_type'] = 'DNA'   # required by GenBank/EMBL writers

Build SeqRecord with a feature

from Bio.SeqFeature import SeqFeature, FeatureLocation

record = SeqRecord(Seq('ATGCGATCGATCG'), id='gene1')
feature = SeqFeature(FeatureLocation(0, 9), type='CDS', qualifiers={'product': ['Example protein']})
record.features.append(feature)

Batch create SeqRecords

sequences = ['ATGC', 'GCTA', 'TTAA']
records = [SeqRecord(Seq(s), id=f'seq_{i}') for i, s in enumerate(sequences)]

Copy a SeqRecord

from copy import deepcopy
new_record = deepcopy(record)   # deep copy; plain assignment shares features/annotations
new_record.id = 'modified_copy'

Join sequences with a linker

combined_seq = seq1 + Seq('NNNN') + seq2
combined_record = SeqRecord(combined_seq, id='combined')

Common Errors

Symptom	Cause	Fix
ImportError: No module named 'Bio.Alphabet' (or cannot import name 'IUPAC')	Bio.Alphabet removed in 1.78	Drop the alphabet argument; set record.annotations['molecule_type'] instead
TypeError: 'Seq' object does not support item assignment	Editing an immutable Seq in place	Use MutableSeq, or rebuild with slicing/concatenation
UndefinedSequenceError on str(seq)/print(seq)	Sequence from a lazy/partial parser (Seq(None, length=n)) has known length but no content	Avoid reading bytes, or guard with except UndefinedSequenceError (subclass of ValueError)
isinstance(seq, str) is False, type-check skips the sequence	Since 1.79 Seq is bytes-backed, not a str subclass	Test isinstance(x, (Seq, MutableSeq)); use str(seq) for text
ValueError: missing molecule_type writing GenBank/EMBL	No molecule_type annotation (or it was dropped by slicing)	Set record.annotations['molecule_type'] = 'DNA' before writing
reverse_complement()/transcribe() returns nonsense, no error	No alphabet validation since 1.78 - a protein/RNA was passed	Track molecule type yourself; only call strand ops on DNA/RNA

Decision Tree

Need to work with sequence data?
├── Only string-like reads (slice, count, find, translate)?
│   └── Use Seq (immutable)
├── Editing individual positions in place?
│   └── Use MutableSeq, then cast back to Seq to write
├── Need metadata (id, description, features, annotations)?
│   └── Use SeqRecord
└── Writing to GenBank/EMBL?
    └── Use SeqRecord with annotations['molecule_type'] set

Related Skills

• sequence-io/read-sequences - Parse files to get SeqRecord objects
• sequence-io/write-sequences - Write SeqRecord objects to files
• transcription-translation - Transform Seq objects (DNA to protein); inherits the no-alphabet-validation trap
• reverse-complement - Get reverse complement of Seq; silent garbage on non-DNA input
• sequence-slicing - Slice and extract from Seq/SeqRecord; 0-based vs 1-based coordinate trap
• database-access/entrez-fetch - Fetch sequences from NCBI as SeqRecords