pyspark.ml.feature.
FeatureHasher
Feature hashing projects a set of categorical or numerical features into a feature vector of specified dimension (typically substantially smaller than that of the original feature space). This is done using the hashing trick (https://en.wikipedia.org/wiki/Feature_hashing) to map features to indices in the feature vector.
The FeatureHasher transformer operates on multiple columns. Each column may contain either numeric or categorical features. Behavior and handling of column data types is as follows:
For numeric features, the hash value of the column name is used to map the feature value to its index in the feature vector. By default, numeric features are not treated as categorical (even when they are integers). To treat them as categorical, specify the relevant columns in categoricalCols.
For categorical features, the hash value of the string “column_name=value” is used to map to the vector index, with an indicator value of 1.0. Thus, categorical features are “one-hot” encoded (similarly to using OneHotEncoder with dropLast=false).
OneHotEncoder
Boolean values are treated in the same way as string columns. That is, boolean features are represented as “column_name=true” or “column_name=false”, with an indicator value of 1.0.
Null (missing) values are ignored (implicitly zero in the resulting feature vector).
Since a simple modulo is used to transform the hash function to a vector index, it is advisable to use a power of two as the numFeatures parameter; otherwise the features will not be mapped evenly to the vector indices.
New in version 2.3.0.
Examples
>>> data = [(2.0, True, "1", "foo"), (3.0, False, "2", "bar")] >>> cols = ["real", "bool", "stringNum", "string"] >>> df = spark.createDataFrame(data, cols) >>> hasher = FeatureHasher() >>> hasher.setInputCols(cols) FeatureHasher... >>> hasher.setOutputCol("features") FeatureHasher... >>> hasher.transform(df).head().features SparseVector(262144, {174475: 2.0, 247670: 1.0, 257907: 1.0, 262126: 1.0}) >>> hasher.setCategoricalCols(["real"]).transform(df).head().features SparseVector(262144, {171257: 1.0, 247670: 1.0, 257907: 1.0, 262126: 1.0}) >>> hasherPath = temp_path + "/hasher" >>> hasher.save(hasherPath) >>> loadedHasher = FeatureHasher.load(hasherPath) >>> loadedHasher.getNumFeatures() == hasher.getNumFeatures() True >>> loadedHasher.transform(df).head().features == hasher.transform(df).head().features True
Methods
clear(param)
clear
Clears a param from the param map if it has been explicitly set.
copy([extra])
copy
Creates a copy of this instance with the same uid and some extra params.
explainParam(param)
explainParam
Explains a single param and returns its name, doc, and optional default value and user-supplied value in a string.
explainParams()
explainParams
Returns the documentation of all params with their optionally default values and user-supplied values.
extractParamMap([extra])
extractParamMap
Extracts the embedded default param values and user-supplied values, and then merges them with extra values from input into a flat param map, where the latter value is used if there exist conflicts, i.e., with ordering: default param values < user-supplied values < extra.
getCategoricalCols()
getCategoricalCols
Gets the value of binary or its default value.
getInputCols()
getInputCols
Gets the value of inputCols or its default value.
getNumFeatures()
getNumFeatures
Gets the value of numFeatures or its default value.
getOrDefault(param)
getOrDefault
Gets the value of a param in the user-supplied param map or its default value.
getOutputCol()
getOutputCol
Gets the value of outputCol or its default value.
getParam(paramName)
getParam
Gets a param by its name.
hasDefault(param)
hasDefault
Checks whether a param has a default value.
hasParam(paramName)
hasParam
Tests whether this instance contains a param with a given (string) name.
isDefined(param)
isDefined
Checks whether a param is explicitly set by user or has a default value.
isSet(param)
isSet
Checks whether a param is explicitly set by user.
load(path)
load
Reads an ML instance from the input path, a shortcut of read().load(path).
read()
read
Returns an MLReader instance for this class.
save(path)
save
Save this ML instance to the given path, a shortcut of ‘write().save(path)’.
set(param, value)
set
Sets a parameter in the embedded param map.
setCategoricalCols(value)
setCategoricalCols
Sets the value of categoricalCols.
categoricalCols
setInputCols(value)
setInputCols
Sets the value of inputCols.
inputCols
setNumFeatures(value)
setNumFeatures
Sets the value of numFeatures.
numFeatures
setOutputCol(value)
setOutputCol
Sets the value of outputCol.
outputCol
setParams(self, \*[, numFeatures, …])
setParams
Sets params for this FeatureHasher.
transform(dataset[, params])
transform
Transforms the input dataset with optional parameters.
write()
write
Returns an MLWriter instance for this ML instance.
Attributes
params
Returns all params ordered by name.
Methods Documentation
Creates a copy of this instance with the same uid and some extra params. This implementation first calls Params.copy and then make a copy of the companion Java pipeline component with extra params. So both the Python wrapper and the Java pipeline component get copied.
Extra parameters to copy to the new instance
JavaParams
Copy of this instance
extra param values
merged param map
Gets the value of a param in the user-supplied param map or its default value. Raises an error if neither is set.
New in version 1.3.0.
pyspark.sql.DataFrame
input dataset
an optional param map that overrides embedded params.
transformed dataset
Attributes Documentation
Returns all params ordered by name. The default implementation uses dir() to get all attributes of type Param.
dir()
Param