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# Lists and Enumerations
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List environments including itemized lists, numbered lists, and description lists with customization options. PyLaTeX provides comprehensive support for LaTeX's list environments, enabling the creation of structured content with automatic numbering, custom symbols, and nested hierarchies.
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## Capabilities
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### Itemized Lists
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The Itemize class creates bulleted lists with default or custom bullet symbols.
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```python { .api }
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class Itemize(List):
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    def __init__(self, *, options=None, **kwargs):
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        """
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        Create an itemized (bulleted) list.
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        Parameters:
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        - options: str, list, or Options, custom list options
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        """
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    def add_item(self, s):
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        """
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        Add an item to the list.
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        Parameters:
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        - s: str or LatexObject, the item content
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        """
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```
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Usage example:
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```python
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from pylatex import Document, Section
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from pylatex.lists import Itemize
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doc = Document()
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with doc.create(Section('Project Requirements')):
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    with doc.create(Itemize()) as itemize:
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        itemize.add_item('User authentication system')
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        itemize.add_item('Database integration')
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        itemize.add_item('RESTful API endpoints')
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        itemize.add_item('Frontend user interface')
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        itemize.add_item('Comprehensive testing suite')
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# Nested lists
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with doc.create(Section('Technical Stack')):
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    with doc.create(Itemize()) as itemize:
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        itemize.add_item('Backend Technologies')
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        # Nested list
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        with itemize.create(Itemize()) as nested:
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            nested.add_item('Python 3.8+')
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            nested.add_item('Django framework')
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            nested.add_item('PostgreSQL database')
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        itemize.add_item('Frontend Technologies')
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        with itemize.create(Itemize()) as nested:
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            nested.add_item('React.js')
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            nested.add_item('TypeScript')
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            nested.add_item('CSS3 with Flexbox')
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```
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### Enumerated Lists
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The Enumerate class creates numbered lists with support for custom numbering schemes and styles.
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```python { .api }
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class Enumerate(List):
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    def __init__(self, enumeration_symbol=None, *, options=None, **kwargs):
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        """
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        Create an enumerated (numbered) list.
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        Parameters:
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        - enumeration_symbol: str, custom enumeration symbol pattern
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        - options: str, list, or Options, custom list options
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        Requires:
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        - enumitem package (automatically added when using enumeration_symbol)
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        """
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    def add_item(self, s):
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        """
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        Add an item to the enumerated list.
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        Parameters:
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        - s: str or LatexObject, the item content
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        """
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```
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Usage example:
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```python
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from pylatex import Document, Section, NoEscape
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from pylatex.lists import Enumerate
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doc = Document()
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with doc.create(Section('Implementation Steps')):
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    # Basic numbered list
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    with doc.create(Enumerate()) as enum:
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        enum.add_item('Design system architecture')
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        enum.add_item('Set up development environment')
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        enum.add_item('Implement core functionality')
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        enum.add_item('Write comprehensive tests')
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        enum.add_item('Deploy to production')
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# Custom numbering styles
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with doc.create(Section('Custom Numbering')):
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    # Roman numerals
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    with doc.create(Enumerate(enumeration_symbol=NoEscape(r'\roman*'))) as enum:
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        enum.add_item('First phase')
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        enum.add_item('Second phase')
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        enum.add_item('Third phase')
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    # Alphabetical
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    with doc.create(Enumerate(enumeration_symbol=NoEscape(r'\alph*)'))) as enum:
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        enum.add_item('Primary objective')
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        enum.add_item('Secondary objective')
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        enum.add_item('Tertiary objective')
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    # Custom format
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    with doc.create(Enumerate(enumeration_symbol=NoEscape(r'Step \arabic*:'))) as enum:
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        enum.add_item('Initialize variables')
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        enum.add_item('Process input data')
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        enum.add_item('Generate output')
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# Nested enumerated lists
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with doc.create(Section('Detailed Workflow')):
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    with doc.create(Enumerate()) as enum:
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        enum.add_item('Data Collection')
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        with enum.create(Enumerate(enumeration_symbol=NoEscape(r'\alph*)'))) as subenum:
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            subenum.add_item('Survey participants')
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            subenum.add_item('Gather measurements')
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            subenum.add_item('Record observations')
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        enum.add_item('Data Analysis')
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        with enum.create(Enumerate(enumeration_symbol=NoEscape(r'\alph*)'))) as subenum:
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            subenum.add_item('Statistical processing')
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            subenum.add_item('Visualization')
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            subenum.add_item('Interpretation')
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```
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### Description Lists
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The Description class creates definition-style lists with terms and their descriptions.
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```python { .api }
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class Description(List):
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    def __init__(self, *, options=None, **kwargs):
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        """
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        Create a description list.
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        Parameters:
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        - options: str, list, or Options, custom list options
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        """
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    def add_item(self, label, s):
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        """
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        Add an item to the description list.
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        Parameters:
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        - label: str, the term or label to define
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        - s: str or LatexObject, the description content
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        """
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```
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Usage example:
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```python
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from pylatex import Document, Section, Command
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from pylatex.lists import Description
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doc = Document()
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with doc.create(Section('Terminology')):
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    with doc.create(Description()) as desc:
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        desc.add_item('API', 'Application Programming Interface - a set of protocols and tools for building software applications')
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        desc.add_item('REST', 'Representational State Transfer - an architectural style for distributed hypermedia systems')
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        desc.add_item('CRUD', 'Create, Read, Update, Delete - the four basic operations of persistent storage')
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        desc.add_item('ORM', 'Object-Relational Mapping - programming technique for converting data between incompatible type systems')
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# Enhanced formatting
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with doc.create(Section('Technical Specifications')):
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    with doc.create(Description()) as desc:
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        desc.add_item(
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            Command('textbf', 'Python'), 
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            'High-level programming language with dynamic semantics and extensive standard library'
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        )
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        desc.add_item(
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            Command('textbf', 'Django'), 
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            'Web framework that encourages rapid development and clean, pragmatic design'
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        )
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        desc.add_item(
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            Command('textbf', 'PostgreSQL'), 
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            'Open-source relational database system with strong reputation for reliability and performance'
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        )
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```
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## Advanced List Features
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### Custom List Styling
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```python
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from pylatex import Document, Section, Package, NoEscape
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from pylatex.lists import Itemize, Enumerate
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from pylatex.base_classes import Options
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doc = Document()
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# Add enumitem package for advanced customization
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doc.packages.append(Package('enumitem'))
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with doc.create(Section('Custom Styled Lists')):
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    # Custom bullet symbols
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    custom_options = Options()
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    custom_options.append(NoEscape(r'label=\textbullet'))
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    with doc.create(Itemize(options=custom_options)) as itemize:
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        itemize.add_item('Custom bullet point')
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        itemize.add_item('Another bullet point')
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    # Compact spacing
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    compact_options = Options()
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    compact_options.append(NoEscape('nosep'))
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    with doc.create(Enumerate(options=compact_options)) as enum:
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        enum.add_item('Tight spacing item 1')
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        enum.add_item('Tight spacing item 2')
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        enum.add_item('Tight spacing item 3')
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    # Custom indentation
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    indent_options = Options()
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    indent_options.append(NoEscape('leftmargin=2cm'))
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    indent_options.append(NoEscape('itemindent=1cm'))
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    with doc.create(Itemize(options=indent_options)) as itemize:
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        itemize.add_item('Custom indented item')
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        itemize.add_item('Another indented item')
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```
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### Multi-Level Lists
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```python
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from pylatex import Document, Section, NoEscape
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from pylatex.lists import Enumerate, Itemize
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doc = Document()
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with doc.create(Section('Project Hierarchy')):
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    with doc.create(Enumerate()) as main_list:
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        main_list.add_item('Frontend Development')
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        with main_list.create(Itemize()) as sub_items:
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            sub_items.add_item('Component Architecture')
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            with sub_items.create(Enumerate(enumeration_symbol=NoEscape(r'\alph*)'))) as subsub:
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                subsub.add_item('Header component')
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                subsub.add_item('Navigation component')
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                subsub.add_item('Footer component')
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            sub_items.add_item('State Management')
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            sub_items.add_item('Routing System')
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        main_list.add_item('Backend Development')
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        with main_list.create(Itemize()) as sub_items:
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            sub_items.add_item('Database Design')
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            sub_items.add_item('API Endpoints')
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            sub_items.add_item('Authentication')
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        main_list.add_item('Testing Strategy')
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```
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### Mixed Content Lists
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```python
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from pylatex import Document, Section, Command, NoEscape
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from pylatex.lists import Enumerate, Description
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from pylatex.math import Math
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doc = Document()
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with doc.create(Section('Mathematical Concepts')):
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    with doc.create(Description()) as desc:
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        # List items with mathematical content
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        desc.add_item(
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            'Quadratic Formula',
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            Math(data=NoEscape(r'x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}'), inline=True)
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        )
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        desc.add_item(
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            'Pythagorean Theorem', 
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            ['For a right triangle: ', Math(data=NoEscape(r'a^2 + b^2 = c^2'), inline=True)]
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        )
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        desc.add_item(
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            Command('textit', 'Euler\'s Identity'),
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            Math(data=NoEscape(r'e^{i\pi} + 1 = 0'), inline=True)
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        )
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# Code examples in lists
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with doc.create(Section('Programming Examples')):
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    with doc.create(Enumerate()) as enum:
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        enum.add_item([
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            'Variable declaration: ',
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            NoEscape(r'\texttt{x = 10}')
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        ])
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        enum.add_item([
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            'Function definition: ',
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            NoEscape(r'\texttt{def func(param):}')
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        ])
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        enum.add_item([
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            'Class instantiation: ',
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            NoEscape(r'\texttt{obj = MyClass()}')
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        ])
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```
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## List Formatting and Styling
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### Spacing and Layout Control
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```python
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from pylatex import Document, Package, Command, NoEscape
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from pylatex.lists import Itemize, Enumerate
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from pylatex.base_classes import Options
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doc = Document()
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doc.packages.append(Package('enumitem'))
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# Global list settings
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doc.preamble.append(Command('setlist', options='itemsep=0.5em,parsep=0pt,partopsep=0pt'))
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with doc.create(Section('Controlled Spacing')):
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    # No extra spacing
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    tight_options = Options(['noitemsep', 'topsep=0pt', 'parsep=0pt'])
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    with doc.create(Itemize(options=tight_options)) as itemize:
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        itemize.add_item('Compact item 1')
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        itemize.add_item('Compact item 2')
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        itemize.add_item('Compact item 3')
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    # Extra spacing
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    spaced_options = Options(['itemsep=1em', 'topsep=1em'])
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    with doc.create(Enumerate(options=spaced_options)) as enum:
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        enum.add_item('Spaced item 1')
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        enum.add_item('Spaced item 2')
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        enum.add_item('Spaced item 3')
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```
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### Custom Symbols and Numbering
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```python
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from pylatex import Document, Section, Package, NoEscape, Command
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from pylatex.lists import Itemize, Enumerate
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doc = Document()
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doc.packages.append(Package('enumitem'))
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doc.packages.append(Package('pifont'))  # For special symbols
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with doc.create(Section('Custom Symbols')):
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    # Special bullet symbols
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    with doc.create(Itemize(enumeration_symbol=NoEscape(r'\ding{43}'))) as itemize:
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        itemize.add_item('Star bullet point')
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        itemize.add_item('Another star point')
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    # Arrow bullets
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    with doc.create(Itemize(enumeration_symbol=NoEscape(r'$\rightarrow$'))) as itemize:
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        itemize.add_item('Arrow bullet point')
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        itemize.add_item('Another arrow point')
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    # Custom numbering with prefixes/suffixes
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    with doc.create(Enumerate(enumeration_symbol=NoEscape(r'Task \arabic*:'))) as enum:
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        enum.add_item('Complete requirements analysis')
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        enum.add_item('Design system architecture')
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        enum.add_item('Implement core features')
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```
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### Continuation and Resuming
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```python
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from pylatex import Document, Section, NoEscape
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from pylatex.lists import Enumerate
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from pylatex.base_classes import Options
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doc = Document()
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with doc.create(Section('Continuing Lists')):
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    # First part of list
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    with doc.create(Enumerate()) as enum:
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        enum.add_item('First item')
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        enum.add_item('Second item')
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        enum.add_item('Third item')
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    doc.append('Some intervening text that breaks the list.')
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    # Resume numbering
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    resume_options = Options(['resume'])
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    with doc.create(Enumerate(options=resume_options)) as enum:
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        enum.add_item('Fourth item (continued)')
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        enum.add_item('Fifth item (continued)')
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```
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## Practical List Applications
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### Task Lists and Checklists
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```python
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from pylatex import Document, Section, Package, NoEscape
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from pylatex.lists import Itemize
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doc = Document()
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doc.packages.append(Package('amssymb'))  # For checkboxes
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with doc.create(Section('Project Checklist')):
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    checkboxes = [
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        (NoEscape(r'$\boxtimes$'), 'Requirements gathering', 'completed'),
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        (NoEscape(r'$\boxtimes$'), 'System design', 'completed'),  
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        (NoEscape(r'$\square$'), 'Implementation', 'pending'),
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        (NoEscape(r'$\square$'), 'Testing', 'pending'),
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        (NoEscape(r'$\square$'), 'Deployment', 'pending')
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    ]
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    with doc.create(Itemize()) as itemize:
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        for checkbox, task, status in checkboxes:
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            itemize.add_item([checkbox, ' ', task])
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```
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### Bibliography-Style Lists
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```python
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from pylatex import Document, Section, Command
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from pylatex.lists import Description
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doc = Document()
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with doc.create(Section('References')):
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    with doc.create(Description()) as desc:
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        desc.add_item(
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            '[1]',
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            'Smith, J. (2023). "Advanced LaTeX Techniques." '
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            + Command('textit', 'Journal of Document Processing') + ', 15(3), 123-145.'
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        )
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        desc.add_item(
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            '[2]',
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            'Johnson, A. & Brown, B. (2022). "Python for Scientific Writing." '
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            + 'Academic Press, New York.'
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        )
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        desc.add_item(
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            '[3]',
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            'Davis, C. (2024). "Automated Document Generation." '
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            + 'Proceedings of the International Conference on Documentation.'
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        )
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```
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### Feature Comparison Lists
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```python
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from pylatex import Document, Section, Command
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from pylatex.lists import Description
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from pylatex.table import Tabular
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doc = Document()
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with doc.create(Section('Feature Comparison')):
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    with doc.create(Description()) as desc:
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        desc.add_item(
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            Command('textbf', 'Basic Plan'),
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            'Up to 5 users, 10GB storage, email support'
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        )
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        desc.add_item(
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            Command('textbf', 'Professional Plan'),
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            'Up to 25 users, 100GB storage, priority support, advanced analytics'
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        )
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        desc.add_item(
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            Command('textbf', 'Enterprise Plan'),
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            'Unlimited users, 1TB storage, dedicated support, custom integrations'
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        )
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```
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## Error Handling and Best Practices
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### Empty List Handling
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Lists in PyLaTeX automatically handle empty content through the `omit_if_empty` attribute:
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```python
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from pylatex.lists import Itemize
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# This will not produce LaTeX output if no items are added
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empty_list = Itemize()
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# empty_list.add_item()  # No items added
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# This ensures the list appears in output
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non_empty_list = Itemize()
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non_empty_list.add_item('At least one item')
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```
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### Package Requirements
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Different list features require specific packages:
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- **Basic lists**: No additional packages
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- **Custom enumeration**: `enumitem` package (automatically added)
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- **Special symbols**: `pifont`, `amssymb`, or `textcomp` packages
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- **Unicode symbols**: May require `fontspec` with XeLaTeX or LuaLaTeX
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```python
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from pylatex import Document, Package
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doc = Document()
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doc.packages.append(Package('enumitem'))
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doc.packages.append(Package('pifont'))
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doc.packages.append(Package('amssymb'))
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```
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The list system in PyLaTeX provides flexible and powerful tools for creating structured content with professional formatting and extensive customization options.