lvmio

Typed reader and writer for the LabVIEW Measurement (.lvm) file format, stdlib only.

Install

pip install lvmio

Usage

Write channels to a spec-faithful .lvm file and read them back:

from lvmio import Channel, LvmFile, Segment, read_lvm, write_lvm

channels = [
    Channel("Voltage", [1.0, 2.0, 3.0], unit="V", delta_x=0.1),
    Channel("Current", [0.5, 0.25, 0.125], unit="A", delta_x=0.1),
]
write_lvm("run.lvm", LvmFile(segments=[Segment(channels)]), operator="lab")

lvm = read_lvm("run.lvm")
first = lvm.segments[0].channels[0]
print(first.name, first.data)  # Voltage [1.0, 2.0, 3.0]

write_lvm also accepts a bare list of Channel objects, a single Segment, or a mapping of name to samples:

write_lvm("quick.lvm", {"x": [1.0, 2.0], "y": [3.0, 4.0]})

Keyword arguments override file header fields: operator, date, time, separator ("Tab" or "Comma"), decimal_separator, and precision (the number of decimal places used for numeric output).

Working with a parsed file

The parsed objects are frozen dataclasses with convenient access:

lvm = read_lvm("run.lvm")
segment = lvm.segments[0]

for channel in segment:  # Segment iterates over its channels
    print(channel.name, len(channel))

voltage = segment["Voltage"]  # access a channel by name
axis = voltage.time_axis()  # [x0, x0 + delta_x, ...], one value per sample
missing = segment.get("absent")  # None when the channel is not present

Empty cells in the data (unconnected channels, or rows without a value) parse to float('nan'), so every channel keeps one value per row and stays aligned with its own time axis.

parse_lvm(text) parses a .lvm string already in memory; read_lvm(path) is the path based convenience that calls it.

Command line

Installing the package adds an lvmio console command:

lvmio header run.lvm            # file metadata, add --json for JSON
lvmio channels run.lvm          # list the channels per segment
lvmio dump run.lvm --channel Voltage --time    # one channel as CSV with its time axis
lvmio dump run.lvm --format json               # all channels of a segment as JSON
cat run.lvm | lvmio header -    # read from standard input

Errors go to standard error and the process exits with status 1. Run lvmio --help or lvmio <command> --help for the full list of options.

Supported subset

The format has a file header block (LabVIEW Measurement, Writer_Version, Separator, Decimal_Separator, Multi_Headings, X_Columns, Time_Pref, Operator, Date, Time) followed by one or more data segments, each with its own channel header and tab-delimited rows.

Covered:

• Tab and comma separators, and a configurable decimal separator (including the decimal comma and scientific notation that European LabVIEW writes).
• Single or multiple segments (Multi_Headings is set from the segment count).
• X_Columns set to One (a shared X axis from X0 and Delta_X) or No. Real No files still carry an empty leading X column, and the reader handles that.
• Per-channel Y_Unit_Label, X_Dimension, X0 and Delta_X.
• The trailing Comment column on the data header, and comments on some rows.
• Empty data cells, which become float('nan').

Behaviour on the layouts LabVIEW writes is locked in by golden fixtures under tests/fixtures/, generated by scripts/make_lvm_fixtures.py (each file says so in its Description header), and cross-checked against the independent lvm_read package.

Out of scope for now: X_Columns set to Multi (one X column per channel) and absolute-time X axes. These parse as plain columns but are not modelled.

Why

LabVIEW produces and consumes .lvm natively with its Write/Read To Measurement File express VIs, and on the Python side lvm-read already reads the format well. Reading, then, is a solved problem. What is missing is the writer: there is no maintained, typed Python package that writes a .lvm file a LabVIEW VI will read back, which otherwise means matching NI's header blocks and tab-delimited segments by hand. lvmio fills that writer gap and keeps a small, correct reader alongside it as a convenience, not as the novelty, so a round trip stays within one dependency-free package. It uses the standard library only.

License

MIT. See LICENSE.

Links

• Website: https://lianes8studio.com
• GitHub: https://github.com/JaviLianes8
• LinkedIn: https://www.linkedin.com/in/jlianes/