Writing TDMS files

npTDMS has rudimentary support for writing TDMS files. The full set of optimisations supported by the TDMS file format for speeding up the writing of files and minimising file size are not implemented by npTDMS, but the basic functionality required to write TDMS files is available.

To write a TDMS file, the TdmsWriter class is used, which should be used as a context manager. The __init__() method accepts the path to the file to create, or a file that has already been opened in binary write mode:

with TdmsWriter("my_file.tdms") as tdms_writer:
    # write data

The write_segment() method is used to write a segment of data to the TDMS file. Because the TDMS file format is designed for streaming data applications, it supports writing data one segment at a time as data becomes available. If you don’t require this functionality you can simple call write_segment once with all of your data.

The write_segment() method takes a list of objects, each of which must be an instance of one of:

Each of RootObject, GroupObject and ChannelObject may optionally have properties associated with them, which are passed into the __init__ method as a dictionary. The data types supported as property values are:

  • Integers
  • Floating point values
  • Strings
  • datetime or numpy datetime64 objects
  • Boolean values

For more control over the data type used to represent a property value, for example to use an unsigned integer type, you can pass an instance of one of the data types from the nptdms.types module.

A complete example of writing a TDMS file with various object types and properties is given below:

from nptdms import TdmsWriter, RootObject, GroupObject, ChannelObject

root_object = RootObject(properties={
    "prop1": "foo",
    "prop2": 3,
})
group_object = GroupObject("group_1", properties={
    "prop1": 1.2345,
    "prop2": False,
})
data = np.array([1.0, 2.0, 3.0, 4.0, 5.0])
channel_object = ChannelObject("group_1", "channel_1", data, properties={})

with TdmsWriter("my_file.tdms") as tdms_writer:
    # Write first segment
    tdms_writer.write_segment([
        root_object,
        group_object,
        channel_object])
    # Write another segment with more data for the same channel
    more_data = np.array([6.0, 7.0, 8.0, 9.0, 10.0])
    channel_object = ChannelObject("group_1", "channel_1", more_data, properties={})
    tdms_writer.write_segment([channel_object])

You could also read a TDMS file and then re-write it by passing TdmsGroup and TdmsChannel instances to the write_segment method. If you want to only copy certain channels for example, you could do something like:

from nptdms import TdmsFile, TdmsWriter, RootObject

original_file = TdmsFile("original_file.tdms")
original_groups = original_file.groups()
original_channels = [chan for group in original_groups for chan in group.channels()]

with TdmsWriter("copied_file.tdms") as copied_file:
    root_object = RootObject(original_file.properties)
    channels_to_copy = [chan for chan in original_channels if include_channel(chan)]
    copied_file.write_segment([root_object] + original_groups + channels_to_copy)

Note that this isn’t suitable for copying channels with scaled data, as the channel data will already have scaling applied.