# Writing Records (Python)
Learn to create MARC records from scratch and write them to files.
## Creating a Record
```python
import mrrc
# Create a new record with a leader
leader = mrrc.Leader()
record = mrrc.Record(leader)
```
## Adding Control Fields
Control fields (001-009) contain unstructured data:
```python
record.add_control_field("001", "12345")
## Adding Data Fields
Data fields have a tag, two indicators, and subfields:
```python
# Create fields inline with subfields= and indicators= kwargs
title = mrrc.Field("245", indicators=["1", "0"], subfields=[
mrrc.Subfield("a", "The Great Gatsby /"),
mrrc.Subfield("c", "F. Scott Fitzgerald."),
])
record.add_field(title)
# Or use positional indicators
author = mrrc.Field("100", "1", " ", subfields=[
mrrc.Subfield("a", "Fitzgerald, F. Scott,"),
mrrc.Subfield("d", "1896-1940."),
])
record.add_field(author)
```
You can also build fields incrementally with `add_subfield()`:
```python
field = mrrc.Field("245", "1", "0")
field.add_subfield("a", "The Great Gatsby /")
field.add_subfield("c", "F. Scott Fitzgerald.")
record.add_field(field)
```
## Adding Multiple Fields
```python
# Add subject headings
subjects = ["Psychological fiction", "Long Island (N.Y.)"]
for subject in subjects:
field = mrrc.Field("650", " ", "0")
field.add_subfield("a", subject)
record.add_field(field)
```
## Configuring the Leader
```python
leader = mrrc.Leader()
# Set common properties
leader.record_status = "n" # New record
leader.record_type = "a" # Language material
leader.bibliographic_level = "m" # Monograph
leader.character_coding = "a" # UTF-8
record = mrrc.Record(leader)
```
## Writing to Files
### Basic Writing
```python
with mrrc.MARCWriter("output.mrc") as writer:
writer.write(record)
```
### Write Multiple Records
```python
with mrrc.MARCWriter("output.mrc") as writer:
for record in records:
writer.write(record)
```
### Read, Modify, Write
```python
# Filter records while copying
with mrrc.MARCWriter("output.mrc") as writer:
for record in mrrc.MARCReader("input.mrc"):
if record.title: # Only write records with titles
writer.write(record)
```
## Modifying Existing Records
Fields obtained from a record are live handles: edits write through to
the record, matching pymarc.
```python
for record in mrrc.MARCReader("input.mrc"):
# Add a local note
note = mrrc.Field("590", " ", " ")
note.add_subfield("a", "Processed by MRRC")
record.add_field(note)
# Edit fields in place
record["245"].indicator1 = "1"
record["245"]["a"] = "Revised title /"
for field in record.get_fields("650"):
field.delete_subfield("9")
# Control fields too
record["005"].data = "20260603120000.0"
```
### Field handles: two differences from pymarc
**Handles are not identical objects.** Each lookup returns a new handle
to the same underlying field. Edits through any handle are visible
through every other, but `record["245"] is record["245"]` is `False` —
don't use `is` or `id()` to compare fields.
**Removal invalidates handles.** pymarc field objects survive record
changes; mrrc handles refuse to operate once any field has been removed
from their record, raising `mrrc.StaleFieldError` instead of guessing
which field you meant. Re-fetch and retry:
```python
subjects = record.get_fields("650")
record.remove_field("440")
subjects[0]["a"] # raises mrrc.StaleFieldError
record.get_fields("650")[0]["a"] # re-fetched: works
```
## Complete Example
```python
#!/usr/bin/env python3
"""Create a MARC record from scratch."""
import mrrc
def create_book_record(title, author, isbn, subjects):
"""Create a bibliographic record for a book."""
# Create leader
leader = mrrc.Leader()
leader.record_status = "n"
leader.record_type = "a"
leader.bibliographic_level = "m"
leader.character_coding = "a"
record = mrrc.Record(leader)
# Control fields
record.add_control_field("001", f"mrrc-{isbn}")
record.add_control_field("008", "200101s2020 xxu||||||||||||||||eng||")
# ISBN
record.add_field(mrrc.Field("020", " ", " ", subfields=[
mrrc.Subfield("a", isbn),
]))
# Author
if author:
record.add_field(mrrc.Field("100", "1", " ", subfields=[
mrrc.Subfield("a", author),
]))
# Title
title_subfields = [mrrc.Subfield("a", title)]
if author:
title_subfields.append(mrrc.Subfield("c", f"by {author}"))
record.add_field(mrrc.Field("245", "1" if author else "0", "0",
subfields=title_subfields))
# Subjects
for subject in subjects:
subj_field = mrrc.Field("650", " ", "0")
subj_field.add_subfield("a", subject)
record.add_field(subj_field)
return record
# Create and write a record
record = create_book_record(
title="Introduction to MARC",
author="Smith, John",
isbn="9780123456789",
subjects=["MARC format", "Library science"]
)
with mrrc.MARCWriter("new_record.mrc") as writer:
writer.write(record)
print("Record created successfully")
```
## Next Steps
- [Reading Records](reading-records.md) - Read and access records
- [Format Conversion](format-conversion.md) - Convert to other formats
- [Python API Reference](../../reference/python-api.md) - Full API documentation