FieldMap (arcpy)

Summary

The FieldMap object provides a field definition and a list of input fields taken from a set of tables or feature classes.

Discussion

The FieldMappings object is a collection of FieldMap objects, and it is used as the parameter value for tools that perform field mapping, such as Merge. The easiest way to work with these objects is to first create a FieldMappings object, then initialize its FieldMap objects by adding the input feature classes or tables that are to be combined. Once all inputs are provided, the FieldMappings object will contain one FieldMap object, or output field, for each unique field name from all the inputs. This list can be modified by adding new fields, altering the properties and/or contents of an output field, or removing any unwanted output fields.

The properties of the FieldMap object include the start and end position of an input text value, so a new output value can be created using a slice of an input value. If a FieldMap object contains multiple input fields from the same table or feature class, each record's values are merged using the mergeRule property. This is a convenient way to join values, such as a street name that is held in one field and a street type that is held in another, for example, Eureka and Street. The joinDelimiter property of FieldMap is used if the mergeRule value Join is specified. Any set of characters, such as a space, can be used as a delimiter. In the above example, this would create a value of Eureka Street.

Syntax

FieldMap ()

Properties

Property	Explanation	Data Type
inputFieldCount (Read Only)	The number of defined input fields.	Integer
joinDelimiter (Read and Write)	A string value used to separate input values from the same table if the output field type is string and the mergeRule is Join.	String
mergeRule (Read and Write)	Defines how values from two or more fields from the same input table are merged into a single output value. Valid choices are as follows: First —The first input value is used. Last —The last input value is used. Join —Merge the values together using the delimiter value to separate the values (only valid if the output field type is text). Min —The smallest input value is used (only valid if the output field type is numeric). Max —The largest input value is used (only valid if the output field type is numeric). Mean —The mean is calculated using the input values (only valid if the output field type is numeric). Median —The median is calculated using the input values (only valid if the output field type is numeric). Sum —The sum is calculated using the input values (only valid if the output field type is numeric). StDev —The standard deviation is calculated using the input values (only valid if the output field type is numeric). Count —The number of values included in statistical calculations. This counts each value except null values.	String
outputField (Read and Write)	The properties of the output field are either set or returned in a Field object.	Field

Method Overview

Method	Explanation
addInputField (table_dataset, field_name, {start_position}, {end_position})	Adds input field to the field map.
findInputFieldIndex (table_dataset, field_name)	Finds an input field from the field map.
getEndTextPosition (index)	Gets end text position from the field map.
getInputFieldName (index)	Gets the name of an input field from the field map, based on the field's index.
getInputTableName (index)	Gets the name of an input table from the field map, based on the table's index.
getStartTextPosition (index)	Gets start text position from the field map.
removeAll ()	Removes all values and creates an empty object.
removeInputField (index)	Removes an input field from the field map.
setEndTextPosition (index, end_position)	Sets end text position for the field map.
setStartTextPosition (index, start_position)	Sets the start text position from the field map.

Methods

addInputField (table_dataset, field_name, {start_position}, {end_position})

Parameter	Explanation	Data Type
table_dataset	The table added to the field map.	String
field_name	The input field name.	String
start_position	The start position of an input text value. (The default value is -1)	Integer
end_position	The end position of an input text value. (The default value is -1)	Integer

findInputFieldIndex (table_dataset, field_name)

Parameter	Explanation	Data Type
table_dataset	The table added to the field map.	String
field_name	The field name.	String

Return Value

Data Type	Explanation
Integer	The index position of the field name.

getEndTextPosition (index)

Parameter	Explanation	Data Type
index	The index position.	Integer

Return Value

Data Type	Explanation
Integer	The end text position.

getInputFieldName (index)

Parameter	Explanation	Data Type
index	The index position.	Integer

Return Value

Data Type	Explanation
String	The input field name.

getInputTableName (index)

Parameter	Explanation	Data Type
index	The index position.	Integer

Return Value

Data Type	Explanation
String	The input table name.

getStartTextPosition (index)

Parameter	Explanation	Data Type
index	The index position.	Integer

Return Value

Data Type	Explanation
Integer	The start text position.

removeAll ()

removeInputField (index)

Parameter	Explanation	Data Type
index	The index position.	Integer

setEndTextPosition (index, end_position)

Parameter	Explanation	Data Type
index	The index position.	Integer
end_position	The end position of an input text value.	Integer

setStartTextPosition (index, start_position)

Parameter	Explanation	Data Type
index	The index position.	Integer
start_position	The start position of an input text value.	Integer

Code Sample

FieldMap example

FieldMap objects are often used when there is a need to merge similar datasets into one, all-encompassing dataset. In this example, the feature class Trees and the shapefile Plants.shp are merged into one feature class: Vegetation. Both original feature classes have two attributes: Type and Diameter. These two attributes must be maintained through the merge.

import arcpy

# Set the workspace
arcpy.env.workspace = 'c:/base'

in_file1 = 'data.gdb/Trees'
in_file2 = 'Plants.shp'
output_file = 'data.gdb/Vegetation'

# Create the required FieldMap and FieldMappings objects
fm_type = arcpy.FieldMap()
fm_diam = arcpy.FieldMap()
fms = arcpy.FieldMappings()

# Get the field names of vegetation type and diameter for both original
# files
tree_type = "Tree_Type"
plant_type = "Plant_Type"

tree_diam = "Tree_Diameter"
plant_diam = "Diameter"

# Add fields to their corresponding FieldMap objects
fm_type.addInputField(in_file1, tree_type)
fm_type.addInputField(in_file2, plant_type)

fm_diam.addInputField(in_file1, tree_diam)
fm_diam.addInputField(in_file2, plant_diam)

# Set the output field properties for both FieldMap objects
type_name = fm_type.outputField
type_name.name = 'Veg_Type'
fm_type.outputField = type_name

diam_name = fm_diam.outputField
diam_name.name = 'Veg_Diam'
fm_diam.outputField = diam_name

# Add the FieldMap objects to the FieldMappings object
fms.addFieldMap(fm_type)
fms.addFieldMap(fm_diam)

# Merge the two feature classes
arcpy.Merge_management([in_file1, in_file2], output_file, fms)

FieldMap example 2

This sample displays the option of using FieldMap objects to merge fields, using the FeatureClassToFeatureClass tool. In this example, a feature class contains information about the number of accidents per intersection in a city. Each year of data is maintained in one field. The user would like to find the average number of accidents in each intersection, without altering the existing table.

import arcpy

# Set the workspace
arcpy.env.workspace = 'c:/base/data.gdb'

in_file = 'AccidentData'
out_file = 'AverageAccidents'

# Create the necessary FieldMap and FieldMappings objects
fm = arcpy.FieldMap()
fm1 = arcpy.FieldMap()
fms = arcpy.FieldMappings()

# Each field with accident data begins with 'Yr' (from Yr2007 to Yr2012).
# The next step loops through each of the fields beginning with 'Yr',
# and adds them to the FieldMap Object
for field in arcpy.ListFields(in_file, 'Yr*'):
    fm.addInputField(in_file, field.name)

# Set the merge rule to find the mean value of all fields in the
# FieldMap object
fm.mergeRule = 'Mean'

# Set properties of the output name.
f_name = fm.outputField
f_name.name = 'AvgAccidents'
f_name.aliasName = 'AvgAccidents'
fm.outputField = f_name

# Add the intersection field to the second FieldMap object
fm1.addInputField(in_file, "Intersection")

# Add both FieldMaps to the FieldMappings Object
fms.addFieldMap(fm)
fms.addFieldMap(fm1)

# Create the output feature class, using the FieldMappings object
arcpy.FeatureClassToFeatureClass_conversion(
    in_file, arcpy.env.workspace, out_file, field_mapping=fms)