Creating Messages

To create a FIX message, first create an instance of the FixMessage class.

messsage = simplefix.FixMessage()

You can then add fields to the message as required. You should add the standard header tags 8, 34, 35, 49, 52, and 56 to all messages, unless you’re deliberately creating a malformed message for testing or similar.

Simple Fields

For most tags, using append_pair() is the easiest way to add a field to the message.

message.append_pair(1, "MC435967")
message.append_pair(54, 1)
message.append_pair(44, 37.0582)

Note that any type of value can be used: it will be explicitly converted to a string before encoding the message.

With a few exceptions, the message retains the order in which fields are added. The exceptions are that fields BeginString (8), BodyLength (9), MsgType (35), and Checksum (10) are encoded in their required locations, regardless of what order they were added to the Message.

Header Fields

The FixMessage class does not distinguish header fields from body fields, with one exception.

To enable fields to be added to the FIX header after body fields have already been added, there’s an optional keyword parameter to the append_pair() method (and other append field methods). If this header parameter is set to True, the field is inserted after any previously added header fields, starting at the beginning of the message.

This is normally used for setting things like MsgSeqNum (34) and SendingTime (52) immediately prior to encoding and sending the message.

message.append_pair(8, "FIX.4.4")
message.append_pair(35, 0)
message.append_pair(49, "SENDER")
message.append_pair(56, "TARGET")
message.append_pair(112, "TR0003692")
message.append_pair(34, 4684, header=True)
message.append_time(52, header=True)

In the example above, field 34 would be inserted at the beginning of the message. After encoding, the order of fields would be: 8, 9, 35, 34, 52, 49, 56, 112, 10.

It’s not necessary, but field 49 and 56 could also be written with header set True, in which case, they’d precede 34 and 52 when encoded.

See append_utc_timestamp() below for details of that method.

Pre-composed Pairs

In some cases, your FIX application might have the message content as pre-composed “tag=value” strings. In this case, as an optimisation, the append_string() or append_strings() methods can be used.

STR_SEQ = ["49=SENDER", "56=TARGET"]

message.append_string(BEGIN_STRING, header=True)
message.append_strings(STR_SEQ, header=True)

As with append_pair(), note that these methods have an optional keyword parameter to ensure that their fields are inserted before body fields.


The FIX protocol defines four time types: UTCTimestamp, UTCTimeOnly, TZTimestamp, and TZTimeOnly. Field values of these types can be added using dedicated functions, avoiding the need to translate and format time values in the application code.

message.append_utc_timestamp(52, precision=6, header=True)
message.append_tz_timestamp(1132, my_datetime)
message.append_utc_time_only(1495, start_time)
message.append_tz_time_only(1079, maturity_time)

The first parameter to these functions is the field’s tag number. The second parameter is optional: if None or not supplied, it defaults to the current time, otherwise it must be a Unix epoch time (like from time.time()), or a datetime instance.

There are two keyword parameters: precision which can be 0 for just seconds, 3 for milliseconds, or 6 for microseconds; and header to insert this field in the header rather than the body.

In addition, there are a set of methods for creating correctly formatted time only values from their components:

message.append_utc_time_only_parts(1495, 7, 0, 0, 0, 0)
message.append_tz_time_only_parts(1079, 20, 0, 0, offset=-300)

As usual, the first parameter to these functions is the field’s tag number. The next three parameters are the hour, minute, and seconds of the time value, followed by optional milliseconds and microseconds values.

The timezone for the TZTimeOnly field is set using an offset value, the number of minutes east of UTC. Thus CET will be offset 60 minutes, and New York offset -240 minutes (four hours west).

Finally, remember that time fields can always be set using a string value if the application already has the value in the correct format or prefers to manage the formatting itself.

Repeating Groups

There is no specific support for creating repeating groups in FixMessages. The count field must be appended first, followed by the group’s member’s fields.

Consequently, it’s not an error to append two fields with the same tag, but note that the count fields are not added automatically.

Data Fields

There are numerous defined fields in the FIX protocol that use the data type. These fields consist of two parts: a length, which must come first, immediately followed by the value field, whose value may include the ASCII SOH character, the ASCII NUL character, and in fact any 8-bit byte value.

To append a data field to a message, the append_data() method can be used. It will correctly add both the length field and the value field.

message.append_data(95, 96, "RAW DATA \x00\x01 VALUE")

which will result in the FIX message content (where | represents the SOH):

95=17|96=RAW DATA \x00\x01 VALUE|