app.py

import streamlit as st

def main():
    st.set_page_config(
        page_title="Learn Streamlit",
        page_icon="📚",
        layout="wide",
        initial_sidebar_state="expanded"
    )
    
    st.title("📚 Streamlit Learning App")
    st.markdown("Learn Streamlit by exploring this interactive app!")
    
    # Sidebar navigation
    st.sidebar.title("Navigation")
    page = st.sidebar.radio(
        "Go to",
        ["Home", "Basic Elements", "Layouts", "Widgets", "Advanced Features"]
    )
    
    if page == "Home":
        show_home()
    elif page == "Basic Elements":
        show_basic_elements()
    elif page == "Layouts":
        show_layouts()
    elif page == "Widgets":
        show_widgets()
    elif page == "Advanced Features":
        show_advanced_features()

def show_home():
    st.header("Welcome to Streamlit Learning!")
    st.markdown("""
    This app demonstrates various Streamlit features while teaching you how to use them.
    
    **How to use this app:**
    1. Select a topic from the sidebar
    2. See live examples of Streamlit code
    3. View the code that creates each example
    4. Try modifying the code in the expanders
    """)
    
    st.image("https://streamlit.io/images/brand/streamlit-logo-secondary-colormark-darktext.png", width=300)
    
    with st.expander("Quick Streamlit Facts"):
        st.markdown("""
        - Streamlit turns data scripts into shareable web apps in minutes
        - No front-end experience required
        - Pure Python
        - Open source (Apache License 2.0)
        - First released in October 2019
        """)

def show_basic_elements():
    st.header("Basic Streamlit Elements")
    
    st.subheader("Text Elements")
    st.code("""
    st.title("This is a title")  # Displays text in title formatting
    st.header("This is a header")  # Section header
    st.subheader("This is a subheader")  # Subsection header
    st.text("This is plain text")  # Fixed-width and monospace text
    st.markdown("This is **markdown**")  # Markdown formatting
    st.write("This is multi-purpose write")  # Can handle multiple arguments
    """)
    
    st.subheader("Display Elements")
    st.code("""
    st.dataframe(pandas_dataframe)  # Interactive dataframe
    st.table(static_data)  # Static table
    st.json({"key": "value"})  # Interactive JSON
    st.metric("Metric", 42, delta=1.2)  # Metric display
    """)
    
    col1, col2 = st.columns(2)
    with col1:
        st.info("This is an info message")
        st.success("This is a success message!")
    with col2:
        st.warning("This is a warning message")
        st.error("This is an error message")

def show_layouts():
    st.header("Streamlit Layouts")
    
    st.subheader("Columns")
    st.code("""
    col1, col2 = st.columns(2)  # Create two columns
    with col1:
        st.write("Content in column 1")
    with col2:
        st.write("Content in column 2")
    """)
    
    # Example of columns
    col1, col2, col3 = st.columns([1, 2, 1])
    with col1:
        st.write("Narrow column")
    with col2:
        st.write("Wide column")
    with col3:
        st.write("Another narrow column")
    
    st.subheader("Expander")
    st.code("""
    with st.expander("Click to expand"):
        st.write("Hidden content that appears when expanded")
    """)
    
    with st.expander("Example Expander"):
        st.write("This content was hidden but now it's visible!")
        st.image("https://via.placeholder.com/150", width=100)
    
    st.subheader("Tabs")
    st.code("""
    tab1, tab2 = st.tabs(["Tab 1", "Tab 2"])
    with tab1:
        st.write("Content for tab 1")
    with tab2:
        st.write("Content for tab 2")
    """)
    
    tab1, tab2 = st.tabs(["First Tab", "Second Tab"])
    with tab1:
        st.write("This is the first tab")
    with tab2:
        st.write("This is the second tab")

def show_widgets():
    st.header("Streamlit Widgets")
    
    st.subheader("Input Widgets")
    st.code("""
    st.button("Click me")  # Button
    st.checkbox("I agree")  # Checkbox
    st.radio("Choose one", ["A", "B", "C"])  # Radio buttons
    st.selectbox("Select", ["Option 1", "Option 2"])  # Dropdown select
    st.multiselect("Choose many", ["A", "B", "C"])  # Multi-select
    st.slider("Slide", 0, 100)  # Slider
    st.select_slider("Slide to select", ["Bad", "Good", "Excellent"])  # Select slider
    st.text_input("Enter text")  # Text input
    st.number_input("Enter number")  # Number input
    st.text_area("Enter long text")  # Text area
    st.date_input("Pick a date")  # Date input
    st.time_input("Pick a time")  # Time input
    st.file_uploader("Upload a file")  # File uploader
    st.color_picker("Pick a color")  # Color picker
    """)
    
    # Interactive widget demo
    col1, col2 = st.columns(2)
    with col1:
        name = st.text_input("What's your name?")
        age = st.slider("How old are you?", 0, 100, 25)
        color = st.color_picker("Pick your favorite color")
    with col2:
        if name:
            st.write(f"Hello, {name}!")
        st.write(f"You're {age} years old")
        st.write("Your favorite color is:")
        st.markdown(f'<div style="width:100%; height:50px; background:{color};"></div>', unsafe_allow_html=True)

def show_advanced_features():
    st.header("Advanced Streamlit Features")
    
    st.subheader("Session State")
    st.code("""
    # Initialize session state
    if 'counter' not in st.session_state:
        st.session_state.counter = 0
    
    # Increment counter
    if st.button('Increment'):
        st.session_state.counter += 1
    
    st.write('Count:', st.session_state.counter)
    """)
    
    # Session state example
    if 'counter' not in st.session_state:
        st.session_state.counter = 0
    
    col1, col2, col3 = st.columns(3)
    with col1:
        if st.button('Increment'):
            st.session_state.counter += 1
    with col2:
        if st.button('Decrement'):
            st.session_state.counter -= 1
    with col3:
        if st.button('Reset'):
            st.session_state.counter = 0
    
    st.write('Current count:', st.session_state.counter)
    
    st.subheader("Caching")
    st.code("""
    @st.cache_data  # Cache data processing functions
    def expensive_computation(a, b):
        time.sleep(2)  # Simulate slow computation
        return a * b
    
    result = expensive_computation(5, 10)
    st.write(result)
    """)
    
    import time
    @st.cache_data
    def expensive_computation(a, b):
        time.sleep(2)  # Simulate slow computation
        return a * b
    
    if st.button("Run expensive computation (cached)"):
        result = expensive_computation(5, 10)
        st.write("Result:", result)
    
    st.info("Notice how the cached version runs instantly after the first run!")

if __name__ == "__main__":
    main()