FoLiA library

This tutorial will introduce the FoLiA Python library, part of PyNLPl. The FoLiA library provides an Application Programming Interface for the reading, creation and manipulation of FoLiA XML documents. The library works under Python 2.7 as well as Python 3, which is the recommended version. The samples in this documentation follow Python 3 conventions.

Prior to reading this document, it is recommended to first read the FoLiA documentation itself and familiarise yourself with the format and underlying paradigm. The FoLiA documentation can be found on the FoLiA website . It is especially important to understand the way FoLiA handles sets/classes, declarations, common attributes such as annotator/annotatortype and the distinction between various kinds of annotation categories such as token annotation and span annotation.

This Python library is also the foundation of the FoLiA Tools collection, which consists of various command line utilities to perform common tasks on FoLiA documents. If you’re merely interested in performing a certain common task, such as a single query or conversion, you might want to check there if it contains is a tool that does what you want already.

Reading FoLiA

Loading a document

Any script that uses FoLiA starts with the import:

from pynlpl.formats import folia

At the basis of any FoLiA processing lies the following class:

Document This is the FoLiA Document and holds all its data in memory.

To read a document from file, instantiate a document as follows:

doc = folia.Document(file="/path/to/document.xml")

This returned Document instance holds the entire document in memory. Note that for large FoLiA documents this may consume quite some memory! If you happened to already have the document content in a string, you can load as follows:

doc = folia.Document(string="<FoLiA ...")

Once you have loaded a document, all data is available for you to read and manipulate as you see fit. We will first illustrate some simple use cases:

To save a document back to the file it was loaded from, we do:

Or we can specify a specific filename:"/tmp/document.xml")


Any content that is in a different XML namespace than the FoLiA namespaces or other supported namespaces (XML, Xlink), will be ignored upon loading and lost when saving.

Printing text

You may want to simply print all (plain) text contained in the document, which is as easy as:


Obtaining the text as a string is done by invoking the document’s Document.text() method:

text = doc.text()

Or alternatively as follows:

text = str(doc)

For any subelement of the document, you can obtain its text in the same fashion as well, by calling its AbstractElement.text() method or by using str(), the only difference is that the former allows for extensive fine tuning using various extra parameters (See AbstractElement.text()).


In Python 2, both str() as well as unicode() return a unicode instance. You may need to append .encode('utf-8') for proper output.


A document instance has an index which you can use to grab any of its elements by ID. Querying using the index proceeds similar to using a python dictionary:

word = doc['example.p.3.s.5.w.1']


Python 2 users will have to do print word.text().encode('utf-8') instead, to ensure non-ascii characters are printed properly.

IDs are unique in the entire document, and preferably even beyond.


All FoLiA elements are derived from AbstractElement and offer an identical interface. To quickly check whether you are dealing with a FoLiA element you can therefore always do the following:

isinstance(word, folia.AbstractElement)

This abstract base element is never instantiated directly. The FoLiA paradigm derives several more abstract base classes which may implement some additional methods or overload some of the original ones:

AbstractElement Abstract base class from which all FoLiA elements are derived.
AbstractStructureElement Abstract element, all structure elements inherit from this class.
AllowTokenAnnotation Elements that allow token annotation (including extended annotation) must inherit from this class
AbstractSpanAnnotation Abstract element, all span annotation elements are derived from this class
AbstractTokenAnnotation Abstract element, all token annotation elements are derived from this class
AbstractAnnotationLayer Annotation layers for Span Annotation are derived from this abstract base class
AbstractTextMarkup Abstract class for text markup elements, elements that appear with the TextContent (t) element.

Obtaining list of elements

The aforementioned index is useful only if you know the ID of the element. This if often not the case, and you will want to iterate through the hierarchy of elements through different means.

If you want to iterate over all of the child elements of a certain element, regardless of what type they are, you can simply do so as follows:

for subelement in element:
    if isinstance(subelement, folia.Sentence):
        print("this is a sentence")
        print("this is something else")

If applied recursively this allows you to traverse the entire element tree, there are however specialised methods available that do this for you.

Select method

There is a generic method available on all elements to select child elements of any desired class. This method is by default applied recursively for most element types:

sentence = doc['example.p.3.s.5.w.1']
words =
for word in words:

The method has a sibling AbstractElement.count(), invoked with the same arguments, which simply counts how many items it finds, without actually returning them:

word = sentence.count(folia.Word)


The select() method and similar high-level methods derived from it, are generators. This implies that the results of the selection are returned one by one in the iteration, as opposed to all stored in memory. This also implies that you can only iterate over it once, we can not do another iteration over the words variable in the above example, unless we reinvoke the select() method to get a new generator. Likewise, we can not do len(words), but have to use the count() method instead.

If you want to have all results in memory in a list, you can simply do the following:

words = list(

The select method is by default recursive, set the third argument to False to make it non-recursive. The second argument can be used for restricting matches to a specific set, a tuple of classes. The recursion will not go into any non-authoritative elements such as alternatives, originals of corrections.

Selection Shortcuts

There are various shortcut methods for select().

For example, you can iterate over all words in the document using Document.words(), or all words under any structural element using AbstractStructureElement.words():

for word in doc.words():

That however gives you one big iteration of words without boundaries. You may more likely want to seek words within sentences, provided the document distinguishes sentences. So we first iterate over all sentences using Document.sentences() and then over the words therein using AbstractStructureElement.words():

for sentence in doc.sentences():
    for word in sentence.words():

Or including paragraphs, assuming the document has them:

for paragraph in doc.paragraphs():
    for sentence in paragraph.sentences():
        for word in sentence.words():


Do be aware that such constructions make presumptions about the structure of the FoLiA document that may not always apply!

All of these shortcut methods also take an index parameter to quickly select a specific item in the sequence:

word = sentence.words(3) #retrieves the fourth word

Structure Annotation Types

The FoLiA library discerns various Python classes for structure annotation, all are subclasses of AbstractStructureElement, which in turn is a subclass of AbstractElement. We list the classes for structure anntoation along with the FoLiA XML tag. Sets and classes can be associated with most of these elements to make them more specific, these are never prescribed by FoLiA. The list of classes is as follows:

Cell A cell in a Row in a Table
Definition Element used in Entry for the portion that provides a definition for the entry.
Division Structure element representing some kind of division.
Entry Represents an entry in a glossary/lexicon/dictionary.
Event Structural element representing events, often used in new media contexts for things such as tweets,chat messages and forum posts.
Example Element that provides an example.
Figure Element for the representation of a graphical figure.
Gap Gap element, represents skipped portions of the text.
Head Head element; a structure element that acts as the header/title of a Division.
Linebreak Line break element, signals a line break.
List Element for enumeration/itemisation.
ListItem Single element in a List.
Note Element used for notes, such as footnotes or warnings or notice blocks.
Paragraph Paragraph element.
Part Generic structure element used to mark a part inside another block.
Quote Quote: a structure element.
Reference A structural element that denotes a reference, internal or external.
Row A row in a Table
Sentence Sentence element.
Table A table consisting of Row elements that in turn consist of Cell elements
Term A term, often used in contect of Entry
TableHead Encapsulated the header of a table, contains Cell elements
Text A full text.
Whitespace Whitespace element, signals a vertical whitespace
Word Word (aka token) element.

The FoLiA documentation explains the exact semantics and use of these in detail. Make sure to consult it to familiarize yourself with how the elements should be used.

FoLiA and this library enforce explicit rules about what elements are allowed in what others. Exceptions will be raised when this is about to be violated.

Common attributes

The FoLiA paradigm features sets and classes as primary means to represent the actual value (class) of an annotation. A set often corresponds to a tagset, such as a set of part-of-speech tags, and a class is one selected value in such a set.

The paradigm furthermore introduces other common attributes to set on annotation elements, such as an identifier, information on the annotator, and more. A full list is provided below:

  • (str) - The unique identifier of the element
  • element.set (str) - The set the element pertains to.
  • element.cls (str) - The assigned class, i.e. the actual value of the annotation, defined in the set. Classes correspond with tagsets in this case of many annotation types. Note that since class is already a reserved keyword in python, the library consistently uses cls everywhere.
  • element.annotator (str) - The name or ID of the annotator who added/modified this element
  • element.annotatortype - The type of annotator, can be either folia.AnnotatorType.MANUAL or folia.AnnotatorType.AUTO
  • element.confidence (float) - A confidence value expressing
  • element.datetime (datetime.datetime) - The date and time when the element was added/modified.
  • element.n (str) - An ordinal label, used for instance in enumerated list contexts, numbered sections, etc..

The following attributes are specific to a speech context:

  • element.src (str) - A URL or filename referring the an audio or video file containing the speech. Access this attribute using the element.speaker_src() method, as it is inheritable from ancestors.
  • element.speaker (str) - The name of ID of the speaker. Access this attribute using the element.speech_speaker() method, as it is inheritable from ancestors.
  • element.begintime (4-tuple) - The time in the above source fragment when the phonetic content of this element starts, this is a (hours, minutes,seconds,milliseconds) tuple.
  • element.endtime (4-tuple) - The time in the above source fragment when the phonetic content of this element ends, this is a (hours, minutes,seconds,milliseconds) tuple.

Attributes that are not available for certain elements, or not set, default to None.


As FoLiA is a format for linguistic annotation, accessing annotation is one of the primary functions of this library. This can be done using the methods AllowTokenAnnotation.annotations() or AllowTokenAnnotation.annotation() that are available on many FoLiA elements. These methods are similar to the method except they will raise a NoSuchAnnotation exception when no such annotation is found. The difference between annotation() and annotations() is that the former will grab only one and raise an exception if there are more between which it can’t disambiguate, whereas the second is a generator, but will still raise an exception if none is found:

for word in doc.words():
        pos = word.annotation(folia.PosAnnotation, 'http://somewhere/CGN')
        lemma = word.annotation(folia.LemmaAnnotation)
        print("Word: ", word)
        print("ID: ",
        print("PoS-tag: " , pos.cls)
        print("PoS Annotator: ", pos.annotator)
        print("Lemma-tag: " , lemma.cls)
    except folia.NoSuchAnnotation:
        print("No PoS or Lemma annotation")

Note that the second argument of AllowTokenAnnotation.annotation(), AllowTokenAnnotation.annotations() or can be used to restrict your selection to a certain set. In the above example we restrict ourselves to Part-of-Speech tags in the CGN set.

Token Annotation Types

The following token annotation elements are available in FoLiA, they are embedded under a structural element (not necessarily a token, despite the name).

DomainAnnotation Domain annotation: an extended token annotation element
PosAnnotation Part-of-Speech annotation: a token annotation element
LangAnnotation Language annotation: an extended token annotation element
LemmaAnnotation Lemma annotation: a token annotation element
SenseAnnotation Sense annotation: a token annotation element
SubjectivityAnnotation Subjectivity annotation/Sentiment analysis: a token annotation element

Text and phonetic annotation

The actual text of an element, or a phonetic textual representation, are also considered annotations themselves.

TextContent Text content element (t), holds text to be associated with whatever element the text content element is a child of.
PhonContent Phonetic content element (ph), holds a phonetic representation to be associated with whatever element the phonetic content element is a child of.

Text is retrieved as string using AbstractElement.text(), or as element using Phonetic content is retrieved as string using AbstractElement.text(), or as element using AbstractElement.textcontent().


These are the only elements for which FoLiA prescribes a default set and a default class (current). This will only be relevant if you work with multiple text layers (current text vs OCRed text for instance) or with corrections of orthography or phonetics.

Span Annotation

FoLiA distinguishes token annotation and span annotation, token annotation is embedded in-line within a structural element, and the annotation therefore pertains to that structural element, whereas span annotation is stored in a stand-off annotation layer outside the element and refers back to it. Span annotation elements typically span over multiple structural elements, they are all subclasses of AbstractSpanAnnotation.

We will discuss three ways of accessing span annotation. As stated, span annotation is contained within an annotation layer (a subclass of AbstractAnnotationLayer) of a certain structure element, often a sentence. In the first way of accessing span annotation, we do everything explicitly: We first obtain the layer, then iterate over the span annotation elements within that layer, and finally iterate over the words to which the span applies. Assume we have a sentence and we want to print all the named entities in it, assuming the entities layer is embedded at sentence level as is conventional:

for layer in
    for entity in
        print(" Entity class=", entity.cls, " words=")
        for word in entity.wrefs():
            print(word, end="")  #print without newline
        print()   #print newline

The AbstractSpanAnnotation.wrefs() method, available on all span annotation elements, will return a list of all words (as well as morphemes and phonemes) over which a span annotation element spans.

This first way is rather verbose. The second way of accessing span annotation takes another approach, using the Word.findspans() method available on Word instances. Here we start from a word and seek span annotations in which that word occurs. Assume we have a word and want to find chunks it occurs in:

for chunk in word.findspans(folia.Chunk):
    print(" Chunk class=", chunk.cls, " words=")
    for word2 in chunk.wrefs(): #print all words in the chunk (of which the word is a part)
        print(word2, end="")

The Word.findspans() method can be called with either the class of a Span Annotation Element, such as Chunk, or with the class of the layer, such as ChunkingLayer.

The third way allows us to look for span elements given an annotation layer and words. In other words, it checks if one or more words form a span. This is an exact match and not a sub-part match as in the previously described method. To do this, we use use the AbstractAnnotationLayer.findspan method, available on all annotation layers:

for span in annotationlayer.findspan(word1,word2):
    print("Class: ", span.cls)
    print("Text: ", span.text()) #same for every span here

Span Annotation Types

This section lists the available Span annotation elements, the layer that contains them is explicitly mentioned as well.

Some of the span annotation elements are complex and take span role elements as children, these are normal span annotation elements that occur on a within another span annotation (of a particular type) and can not be used standalone.

FoLiA distinguishes the following span annotation elements:

Chunk Chunk element, span annotation element to be used in ChunkingLayer
CoreferenceChain Coreference chain.
Dependency Span annotation element to encode dependency relations
Entity Entity element, for entities such as named entities, multi-word expressions, temporal entities.
Observation Observation.
Predicate Predicate, used within SemanticRolesLayer, takes SemanticRole annotations as children, but has its own annotation type and separate declaration
Sentiment Sentiment.
Statement Statement.
SyntacticUnit Syntactic Unit, span annotation element to be used in SyntaxLayer
SemanticRole Semantic Role
TimeSegment A time segment

These are placed in the following annotation layers:

ChunkingLayer Chunking Layer: Annotation layer for Chunk span annotation elements
CoreferenceLayer Syntax Layer: Annotation layer for SyntacticUnit span annotation elements
DependenciesLayer Dependencies Layer: Annotation layer for Dependency span annotation elements.
EntitiesLayer Entities Layer: Annotation layer for Entity span annotation elements.
ObservationLayer Observation Layer: Annotation layer for Observation span annotation elements.
SentimentLayer Sentiment Layer: Annotation layer for Sentiment span annotation elements, used for sentiment analysis.
StatementLayer Statement Layer: Annotation layer for Statement span annotation elements, used for attribution annotation.
SyntaxLayer Syntax Layer: Annotation layer for SyntacticUnit span annotation elements
SemanticRolesLayer Syntax Layer: Annotation layer for SemanticRole span annotation elements
TimingLayer Timing layer: Annotation layer for TimeSegment span annotation elements.

Some span annotation elements take span roles, depending on their type:

CoreferenceLink Coreference link.
DependencyDependent Span role element that marks the dependent in a dependency relation.
Headspan The headspan role is used to mark the head of a span annotation.

Editing FoLiA

Creating a new document

Creating a new FoliA document, rather than loading an existing one from file, is done by explicitly providing the ID for the new document in the Document constructor:

doc = folia.Document(id='example')


Whenever you add a new type of annotation, or a different set, to a FoLiA document, you have to first declare it. This is done using the Document.declare() method. It takes as arguments the annotation type, the set, and you can optionally pass keyword arguments to annotator= and annotatortype= to set defaults.

An example for Part-of-Speech annotation:

doc.declare(folia.PosAnnotation, 'http://somewhere/brown-tag-set')

An example with a default annotator:

doc.declare(folia.PosAnnotation, 'http://somewhere/brown-tag-set', annotator='proycon', annotatortype=folia.AnnotatorType.MANUAL)

Any additional sets for Part-of-Speech would have to be explicitly declared as well. To check if a particular annotation type and set is declared, use the Document.declared() method.

Adding structure

Assuming we begin with an empty document, we should first add a Text element. Then we can add paragraphs, sentences, or other structural elements. The AbstractElement.add() method adds new children to an element:

text = doc.add(folia.Text)
paragraph = text.add(folia.Paragraph)
sentence = paragraph.add(folia.Sentence)
sentence.add(folia.Word, 'This')
sentence.add(folia.Word, 'is')
sentence.add(folia.Word, 'a')
sentence.add(folia.Word, 'test')
sentence.add(folia.Word, '.')


The AbstractElement.add() method is actually a wrapper around AbstractElement.append(), which takes the exact same arguments. It performs extra checks and works for both span annotation as well as token annotation. Using append() will be faster though.

Adding annotations

Adding annotations, or any elements for that matter, is done using the AbstractElement.add() method on the intended parent element. We assume that the annotations we add have already been properly declared, otherwise an exception will be raised as soon as add() is called. Let’s build on the previous example:

#First we grab the fourth word, 'test', from the sentence
word = sentence.words(3)

#Add Part-of-Speech tag
word.add(folia.PosAnnotation, set='brown-tagset',cls='n')

#Add lemma
lemma.add(folia.LemmaAnnotation, cls='test')

Note that in the above examples, the add() method takes a class as first argument, and subsequently takes keyword arguments that will be passed to the classes’ constructor.

A second way of using AbstractElement.add() is by simply passing a fully instantiated child element, thus constructing it prior to adding. The following is equivalent to the above example, as the previous method is merely a shortcut for convenience:

#First we grab the fourth word, 'test', from the sentence
word = sentence.words(3)

#Add Part-of-Speech tag
word.add( folia.PosAnnotation(doc, set='brown-tagset',cls='n') )

#Add lemma
lemma.add( folia.LemmaAnnotation(doc , cls='test') )

The AbstractElement.add() method always returns that which was added, allowing it to be chained.

In the above example we first explicitly instantiate a PosAnnotation and a LemmaAnnotation. Instantiation of any FoLiA element (always Python class subclassed off AbstractElement) follows the following pattern:

Class(document, *children, **kwargs)


See AbstractElement.__init__() for all details on construction

Note that the document has to be passed explicitly as first argument to the constructor.

The common attributes are set using equally named keyword arguments:

  • id=
  • cls=
  • set=
  • annotator=
  • annotatortype=
  • confidence=
  • src=
  • speaker=
  • begintime=
  • endtime=

Not all attributes are allowed for all elements, and certain attributes are required for certain elements. ValueError exceptions will be raised when these constraints are not met.

Instead of setting id. you can also set the keyword argument generate_id_in and pass it another element, an ID will be automatically generated, based on the ID of the element passed. When you use the first method of adding elements, instantiation with generate_id_in will take place automatically behind the scenes when applicable and when id is not explicitly set.

Any extra non-keyword arguments should be FoLiA elements and will be appended as the contents of the element, i.e. the children or subelements. Instead of using non-keyword arguments, you can also use the keyword argument content and pass a list. This is a shortcut made merely for convenience, as Python obliges all non-keyword arguments to come before the keyword-arguments, which if often aesthetically unpleasing for our purposes. Example of this use case will be shown in the next section.

Adding span annotation

Adding span annotation is easy with the FoLiA library. As you know, span annotation uses a stand-off annotation embedded in annotation layers. These layers are in turn embedded in structural elements such as sentences. However, the AbstractElement.add() method abstracts over this. Consider the following example of a named entity:

doc.declare(folia.Entity, "")

sentence = text.add(folia.Sentence)
sentence.add(folia.Word, 'I',id='example.s.1.w.1')
sentence.add(folia.Word, 'saw',id='example.s.1.w.2')
sentence.add(folia.Word, 'the',id='example.s.1.w.3')
word = sentence.add(folia.Word, 'Dalai',id='example.s.1.w.4')
word2 =sentence.add(folia.Word, 'Lama',id='example.s.1.w.5')
sentence.add(folia.Word, '.', id='example.s.1.w.6')

word.add(folia.Entity, word, word2, cls="per")

To make references to the words, we simply pass the word instances and use the document’s index to obtain them. Note also that passing a list using the keyword argument contents is wholly equivalent to passing the non-keyword arguments separately:

word.add(folia.Entity, cls="per", contents=[word,word2])

In the next example we do things more explicitly. We first create a sentence and then add a syntax parse, consisting of nested elements:

doc.declare(folia.SyntaxLayer, 'some-syntax-set')

sentence = text.add(folia.Sentence)
sentence.add(folia.Word, 'The',id='example.s.1.w.1')
sentence.add(folia.Word, 'boy',id='example.s.1.w.2')
sentence.add(folia.Word, 'pets',id='example.s.1.w.3')
sentence.add(folia.Word, 'the',id='example.s.1.w.4')
sentence.add(folia.Word, 'cat',id='example.s.1.w.5')
sentence.add(folia.Word, '.', id='example.s.1.w.6')

#Adding Syntax Layer
layer = sentence.add(folia.SyntaxLayer)

#Adding Syntactic Units
    folia.SyntacticUnit(self.doc, cls='s', contents=[
        folia.SyntacticUnit(self.doc, cls='np', contents=[
            folia.SyntacticUnit(self.doc, self.doc['example.s.1.w.1'], cls='det'),
            folia.SyntacticUnit(self.doc, self.doc['example.s.1.w.2'], cls='n'),
        folia.SyntacticUnit(self.doc, cls='vp', contents=[
            folia.SyntacticUnit(self.doc, self.doc['example.s.1.w.3'], cls='v')
                folia.SyntacticUnit(self.doc, cls='np', contents=[
                    folia.SyntacticUnit(self.doc, self.doc['example.s.1.w.4'], cls='det'),
                    folia.SyntacticUnit(self.doc, self.doc['example.s.1.w.5'], cls='n'),
        folia.SyntacticUnit(self.doc, self.doc['example.s.1.w.6'], cls='fin')


The lower-level AbstractElement.append() method would have had the same effect in the above syntax tree sample.

Deleting annotations

Any element can be deleted by calling the AbstractElement.remove() method on its parent. Suppose we want to delete word:


Copying annotations

A deep copy can be made of any element by calling its AbstractElement.copy() method:

word2 = word.copy()

The copy will be without parent and document. If you intend to associate a copy with a new document, then copy as follows instead:

word2 = word.copy(newdoc)

If you intend to attach the copy somewhere in the same document, you may want to add a suffix for any identifiers in its scope, since duplicate identifiers are not allowed and would raise an exception. This can be specified as the second argument:

word2 = word.copy(doc, ".copy")

Searching in a FoLiA document

If you have loaded a FoLiA document into memory, you may want to search for a particular annotations. You can of course loop over all structural and annotation elements using, AllowTokenAnnotation.annotation() and AllowTokenAnnotation.annotations(). Additionally, Word.findspans() and AbstractAnnotationLayer.findspan() are useful methods of finding span annotations covering particular words, whereas AbstractSpanAnnotation.wrefs() does the reverse and finds the words for a given span annotation element. In addition to these main methods of navigation and selection, there is higher-level function available for searching, this uses the FoLiA Query Language (FQL) or the Corpus Query Language (CQL).

These two languages are part of separate libraries that need to be imported:

from pynlpl.formats import fql, cql

Corpus Query Language (CQL)

CQL is the easier-language of the two and most suitable for corpus searching. It is, however, less flexible than FQL, which is designed specifically for FoLiA and can not just query, but also manipulate FoLiA documents in great detail.

CQL was developed for the IMS Corpus Workbench, at Stuttgart Univeristy, and is implemented in Sketch Engine, who provide good CQL documentation.

CQL has to be converted to FQL first, which is then executed on the given document. This is a simple example querying for the word “house”:

doc = folia.Document(file="/path/to/some/document.folia.xml")
query = fql.Query(cql.cql2fql('"house"'))
for word in query(doc):
    print(word) #these will be folia.Word instances (all matching house)

Multiple words can be queried:

query = fql.Query(cql.cql2fql('"the" "big" "house"'))
for word1,word2,word3 in query(doc):
    print(word1, word2,word3)

Queries may contain wildcard expressions to match multiple text patterns. Gaps can be specified using []. The following will match any three word combination starting with the and ending with something that starts with house. It will thus match things like “the big house” or “the small household”:

query = fql.Query(cql.cql2fql('"the" [] "house.*"'))
for word1,word2,word3 in query(doc):

We can make the gap optional with a question mark, it can be lenghtened with + or * , like regular expressions:

query = fql.Query(cql.cql2fql('"the" []? "house.*"'))
for match in query(doc):
    print("We matched ", len(match), " words")

Querying is not limited to text, but all of FoLiA’s annotations can be used. To force our gap consist of one or more adjectives, we do:

query = fql.Query(cql.cql2fql('"the" [ pos = "a" ]+ "house.*"'))
for match in query(doc):

The original CQL attribute here is tag rather than pos, this can be used too. In addition, all FoLiA element types can be used! Just use their FoLiA tagname.

Consult the CQL documentation for more. Do note that CQL is very word/token centered, for searching other types of elements, use FQL instead.

FoLiA Query Language (FQL)

FQL is documented here, a full overview is beyond the scope of this documentation. We will just introduce some basic selection queries so you can develop an initial impression of the language’s abilities.

All FQL processing is done via the following class, as already seen in the previous section:

Query This class represents an FQL query.

Selecting a word with a particular text is done as follows:

query = fql.Query('SELECT w WHERE text = "house"')
for word in query(doc):
    print(word)  #this will be an instance of folia.Word

Regular expression matching can be done using the MATCHES operator:

query = fql.Query('SELECT w WHERE text MATCHES "^house.*$"')
for word in query(doc):

The classes of other annotation types can be easily queried as follows:

query = fql.Query('SELECT w WHERE :pos = "v"' AND :lemma = "be"')
for word in query(doc):

You can constrain your queries to a particular target selection using the FOR keyword:

query = fql.Query('SELECT w WHERE text MATCHES "^house.*$" FOR s WHERE text CONTAINS "sell"')
for word in query(doc):

This construction also allows you to select the actual annotations. To select all people (a named entity) for words that are not John:

query = fql.Query('SELECT entity WHERE class = "person" FOR w WHERE text != "John"')
for entity in query(doc):
    print(entity) #this will be an instance of folia.Entity

FOR statement may be chained, and Explicit IDs can be passed using the ID keyword:

query = fql.Query('SELECT entity WHERE class = "person" FOR w WHERE text != "John" FOR div ID "section.21"')
for entity in query(doc):

Sets are specified using the OF keyword, it can be omitted if there is only one for the annotation type, but will be required otherwise:

query = fql.Query('SELECT su OF "http://some/syntax/set" WHERE class = "np"')
for su in query(doc):
    print(su) #this will be an instance of folia.SyntacticUnit

We have just covered the SELECT keyword, FQL has other keywords for manipulating documents, such as EDIT, ADD, APPEND and PREPEND.


Consult the FQL documentation at for further documentation on the language.

Streaming Reader

Throughout this tutorial you have seen the Document class as a means of reading FoLiA documents. This class always loads the entire document in memory, which can be a considerable resource demand. The following class provides an alternative to loading FoLiA documents:

Reader Streaming FoLiA reader.

It does not load the entire document in memory but merely returns the elements you are interested in. This results in far less memory usage and also provides a speed-up.

A reader is constructed as follows, the second argument is the class of the element you want:

reader = folia.Reader("my.folia.xml", folia.Word)
for word in reader:

Higher-Order Annotations

Text Markup

FoLiA has a number of text markup elements, these appear within the TextContent (t) element, iterating over the element of a TextContent element will first and foremost produce strings, but also uncover these markup elements when present. The following markup types exists:

TextMarkupGap Markup element to mark gaps in text content (TextContent)
TextMarkupString Markup element to mark arbitrary substrings in text content (TextContent)
TextMarkupStyle Markup element to style text content (TextContent), e.g.
TextMarkupCorrection Markup element to mark corrections in text content (TextContent).
TextMarkupError Markup element to mark gaps in text content (TextContent)


Features allow a second-order annotation by adding the ability to assign properties and values to any of the existing annotation elements. They follow the set/class paradigm by adding the notion of a subset and class relative to this subset. The AbstractElement.feat() method provides a shortcut that can be used on any annotation element to obtain the class of the feature, given a subset. To illustrate the concept, take a look at part of speech annotation with some features:

pos = word.annotation(folia.PosAnnotation)
if pos.cls = "n":
    if pos.feat('number') == 'plural':
        print("We have a plural noun!")
    elif pos.feat('number') == 'singular':
        print("We have a singular noun!")

The AbstractElement.feat() method will return an exception when the feature does not exist. Note that the actual subset and class values are defined by the set and not FoLiA itself! They are therefore fictitious in the above example.

The Python class for features is Feature, in the following example we add a feature:

pos.add(folia.Feature, subset="gender", cls="f")

Although FoLiA does not define any sets nor subsets. Some annotation types do come with some associated subsets, their use is never mandatory. The advantage is that these associated subsets can be directly used as an XML attribute in the FoLiA document. The FoLiA library provides extra classes, all subclassed off Feature for these:

Feature Feature elements can be used to associate subsets and subclasses with almost any
SynsetFeature Synset feature, to be used within Sense
ActorFeature Actor feature, to be used within Event
BegindatetimeFeature Begindatetime feature, to be used within Event
EnddatetimeFeature Enddatetime feature, to be used within Event


A key feature of FoLiA is its ability to make explicit alternative annotations, for token annotations, the Alternative (alt) class is used to this end. Alternative annotations are embedded in this structure. This implies the annotation is not authoritative, but is merely an alternative to the actual annotation (if any). Alternatives may typically occur in larger numbers, representing a distribution each with a confidence value (not mandatory). Each alternative is wrapped in its own Alternative element, as multiple elements inside a single alternative are considered dependent and part of the same alternative. Combining multiple annotation in one alternative makes sense for mixed annotation types, where for instance a pos tag alternative is tied to a particular lemma:

alt = word.add(folia.Alternative)
alt.add(folia.PosAnnotation, set='brown-tagset',cls='n',confidence=0.5)
alt = word.add(folia.Alternative)   #note that we reassign the variable!
alt.add(folia.PosAnnotation, set='brown-tagset',cls='a',confidence=0.3)
alt = word.add(folia.Alternative)
alt.add(folia.PosAnnotation, set='brown-tagset',cls='v',confidence=0.2)

Span annotation elements have a different mechanism for alternatives, for those the entire annotation layer is embedded in a AlternativeLayers element. This element should be repeated for every type, unless the layers it describeds are dependent on it eachother:

alt = sentence.add(folia.AlternativeLayers)
layer = alt.add(folia.Entities)
entity = layer.add(folia.Entity, word1,word2,cls="person", confidence=0.3)

Because the alternative annotations are non-authoritative, normal selection methods such as select() and annotations() will never yield them, unless explicitly told to do so. For this reason, there is an alternatives() method on structure elements, for the first category of alternatives.

In summary, a list of the two relevant classes for alternatives:

Alternative Element grouping alternative token annotation(s).
AlternativeLayers Element grouping alternative subtoken annotation(s).


Corrections are one of the most complex annotation types in FoLiA. Corrections can be applied not just over text, but over any type of structure annotation, token annotation or span annotation. Corrections explicitly preserve the original, and recursively so if corrections are done over other corrections.

Despite their complexity, the library treats correction transparently. Whenever you query for a particular element, and it is part of a correction, you get the corrected version rather than the original. The original is always non-authoritative and normal selection methods will ignore it.

If you want to deal with correction, you have to explicitly handle the Correction element. If an element is part of a correction, its AbstractElement.incorrection() method will give the correction element, if not, it will return None:

pos = word.annotation(folia.PosAnnotation)
correction = pos.incorrection()
if correction:
    if correction.hasoriginal():
        originalpos = correction.original(0) #assuming it's the only element as is customary
        #originalpos will be an instance of folia.PosAnnotation
        print("The original pos was", originalpos.cls)

Corrections themselves carry a class too, indicating the type of correction (defined by the set used and not by FoLiA).

Besides Correction.original(), corrections distinguish three other types, (the corrected version), Correction.current() (the current uncorrected version) and Correction.suggestions() (a suggestion for correction), the former two and latter two usually form pairs, current() and new() can never be used together. Of suggestions(index) there may be multiple, hence the index argument. These return, respectively, instances of Original, folia.New, folia.Current and folia.Suggestion.

Adding a correction can be done explicitly:

wrongpos = word.annotation(folia.PosAnnotation)
word.add(folia.Correction, folia.New(doc, folia.PosAnnotation(doc, cls="n")) , folia.Original(doc, wrongpos), cls="misclassified")

Let’s settle for a suggestion rather than an actual correction:

wrongpos = word.annotation(folia.PosAnnotation)
word.add(folia.Correction, folia.Suggestion(doc, folia.PosAnnotation(doc, cls="n")), cls="misclassified")

In some instances, when correcting text or structural elements, New may be empty, which would correspond to an deletion. Similarly, Original may be empty, corresponding to an insertion.

The use of Current is reserved for use with structure elements, such as words, in combination with suggestions. The structure elements then have to be embedded in Current. This situation arises for instance when making suggestions for a merge or split.

Here is a list of all relevant classes for corrections:

Correction Corrections are one of the most complex annotation types in FoLiA.
Current Used in the context of Correction to encapsulate the currently authoritative annotations.
ErrorDetection The ErrorDetection element is used to signal the presence of errors in a structural element.
Original Used in the context of Correction to encapsulate the original annotations prior to correction.
Suggestion Suggestions are used in the context of Correction, but rather than provide an authoritative correction, it instead offers a suggestion for correction.


Alignments are used to make reference to external documents. It concerns references as annotation rather than references which are explicitly part of the text, such as hyperlinks and Reference.

The following elements are relevant for alignments:

Alignment The Alignment element is a form of higher-order annotation taht is used to point to an external resource.
AlignReference The AlignReference element is used to point to specific elements inside the aligned source.

Descriptions, Metrics

FoLiA allows arbitrary descriptions to be assigned with any element. It also allows assigning metrics to any annotation, which consist of a key/value pair that often express a quantivative or qualitative measure. This is accomplished, respectively, with the following element classes:

Description Description is an element that can be used to associate a description with almost any other FoLiA element
Metric Metric elements provide a key/value pair to allow the annotation of any kind of metric with any kind of annotation element.


FoLiA can be used with a variety of more advanced metadata schemes (e.g. Dublin Core, CMDI). If this is too much, you can use its own simple native metadata facility, a simple key value store . After instantiation of a Document, the metadata can be accessed through the metadata attribute, which behaves like a Python dictionary:

doc = folia.Document(file="/path/to/document.xml")
doc.metadata['language'] = "en"