Executes a user-supplied Python script to manipulate features.

Note: Python is a programming language external to FME. For documentation on creating Python scripts, visit The Python Foundation.

Using Python to perform arbitrary operations on features is a powerful aspect of Workbench. However, the logic introduced into a workspace is less visible and can therefore be more difficult to maintain than logic built using Workbench’s built-in transformers. It is recommended that other transformers be used when possible instead of Python scripts.

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Typical Uses

  • Tasks where a transformer is not available
  • Using external modules for processing
  • Performing complex manipulations on list attributes

How does it work?

The PythonCaller executes a Python script to manipulate features.

When a specialized task is required, such as custom statistical analysis of an attribute, but Workbench does not provide a transformer suited to the task, a Python script can perform specialized and complex operations on a feature's geometry, attributes, and coordinate system.

Access is provided via the FME Objects Python API.

Interface Paradigm

The PythonCaller can interface with a Python script in two different ways - by a function or by a class.

  • Use the Function Interface when you intend to process a single feature at a time.
  • Use the Class Interface for more flexibility.

The Class Interface is useful when you want to operate on a group of features together, such as collecting all the features received and then outputting them in a specific sort order. Another common use case is to accumulate all the features, perform an operation on the whole set, and then output all of the features with a calculated value as a new attribute.

Function Interface

The PythonCaller will call the Python function with exactly one argument: an FMEFeature object.

The function will be called with each FMEFeature that comes into the input port. This feature will then continue through the workspace pipeline via the output port.

The function’s return value will be ignored by the PythonCaller. Any raised exception will terminate the translation. Any raised FMEException will be logged as an ERROR and will terminate the translation.

The example below adds a string attribute to each feature and sets it to the current time:

import fmeobjects
import time
def timestampFeature(feature):
curTime = time.ctime(time.time())
feature.setAttribute("timestamp", curTime)

Class Interface

The PythonCaller will call the following methods on the class:

  • __init__() - Called once, whether or not any features are processed.
  • input() - Called for each FMEFeature that comes into the input port.
  • close() - Called once, after all features are processed (when no more FMEFeatures remain). If no features are processed, the close() method will still be called.

Features that need to continue through the workspace for further processing must be explicitly written out using the pyoutput() method.

The class interface can operate on a group of features, instead of processing incoming features one at a time. This is done by storing incoming features in a list, then processing them all at once before being output.

The example below calculates the total area of all the features processed and then outputs all the features with a new attribute containing the total area:

import fmeobjects
class FeatureProcessor(object):
def __init__(self):
self.featureList = []
self.totalArea = 0.0
def input(self,feature):
self.totalArea += feature.getGeometry().getArea()
def close(self):
 for feature in self.featureList:
 feature.setAttribute("total_area", self.totalArea)

Script Editing

A PythonCaller transformer can call scripts that are stored in the transformer itself or scripts that are stored globally for the entire workspace:

  • To store a Python script with a specific PythonCaller transformer, use the Python Script parameter in the transformer.
  • To store a Python script globally, click the Advanced Workspace Parameter in the Navigator, and double-click Startup Python Script. Storing scripts globally has the advantage of keeping the Python logic centralized, which makes editing and maintenance easier. This is useful when there are multiple PythonCaller transformers throughout the workspace that use the same script. For more information, see Startup and Shutdown Python Scripts in the FME Workbench help.

FME can access .py modules that are stored on the file system, including modules in external Python libraries. Use the Python "import" command to load these modules. FME will search both the standard Python module locations and the workspace location to find the module to be imported.


Input Ports

Output Ports


Editing Transformer Parameters

Using a set of menu options, transformer parameters can be assigned by referencing other elements in the workspace. More advanced functions, such as an advanced editor and an arithmetic editor, are also available in some transformers. To access a menu of these options, click beside the applicable parameter. For more information, see Transformer Parameter Menu Options.

Defining Values

There are several ways to define a value for use in a Transformer. The simplest is to simply type in a value or string, which can include functions of various types such as attribute references, math and string functions, and workspace parameters. There are a number of tools and shortcuts that can assist in constructing values, generally available from the drop-down context menu adjacent to the value field.


Processing Behavior

Feature-Based or Group-Based, conditional on Python script

Feature Holding

Conditional on Python script

FME Licensing Level FME Base Edition and above


FME Community

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Search for all results about the PythonCaller on the FME Community.


Examples may contain information licensed under the Open Government Licence – Vancouver