# Source: https://docs.streamlit.io/get-started/fundamentals/main-concepts

# Basic concepts of Streamlit

Working with Streamlit is simple. First you sprinkle a few Streamlit commands into a normal Python script, then you run it with `streamlit run`: 

```bash
streamlit run your_script.py [-- script args]
```

As soon as you run the script as shown above, a local Streamlit server will spin up and your app will open in a new tab in your default web browser. The app is your canvas, where you'll draw charts, text, widgets, tables, and more.

What gets drawn in the app is up to you. For example, [st.text](/develop/api-reference/text/st.text) writes raw text to your app, and [st.line_chart](/develop/api-reference/charts/st.line_chart) draws — you guessed it — a line chart. Refer to our [API documentation](/develop/api-reference) to see all commands that are available to you.

When passing your script some custom arguments, they must be passed after two dashes. Otherwise the arguments get interpreted as arguments to Streamlit itself.

Another way of running Streamlit is to run it as a Python module. This can be useful when configuring an IDE like PyCharm to work with Streamlit:

```bash
# Running
python -m streamlit run your_script.py

# is equivalent to:
streamlit run your_script.py
```

You can also pass a URL to `streamlit run`! This is great when combined with GitHub Gists. For example:

```bash
streamlit run https://raw.githubusercontent.com/streamlit/demo-uber-nyc-pickups/master/streamlit_app.py
```

## Development flow

Every time you want to update your app, save the source file. When you do that, Streamlit detects if there is a change and asks you whether you want to rerun your app. Choose "Always rerun" at the top-right of your screen to automatically update your app every time you change its source code.

This allows you to work in a fast interactive loop: you type some code, save it, try it out live, then type some more code, save it, try it out, and so on until you're happy with the results. This tight loop between coding and viewing results live is one of the ways Streamlit makes your life easier.

While developing a Streamlit app, it's recommended to lay out your editor and browser windows side by side, so the code and the app can be seen at the same time. Give it a try!

As of Streamlit version 1.10.0 and higher, Streamlit apps cannot be run from the root directory of Linux distributions. If you try to run a Streamlit app from the root directory, Streamlit will throw a `FileNotFoundError: [Errno 2] No such file or directory` error. For more information, see GitHub issue [#5239](https://github.com/streamlit/streamlit/issues/5239).

If you are using Streamlit version 1.10.0 or higher, your main script should live in a directory other than the root directory. When using Docker, you can use the `WORKDIR` command to specify the directory where your main script lives. For an example of how to do this, read [Create a Dockerfile](/deploy/tutorials/docker#create-a-dockerfile).

## Data flow

Streamlit's architecture allows you to write apps the same way you write plain Python scripts. To unlock this, Streamlit apps have a unique data flow: any time something must be updated on the screen, Streamlit reruns your entire Python script from top to bottom.

This can happen in two situations:

- Whenever you modify your app's source code.
- Whenever a user interacts with widgets in the app. For example, when dragging a slider, entering text in an input box, or clicking a button.

Whenever a callback is passed to a widget via the `on_change` (or `on_click`) parameter, the callback will always run before the rest of your script. For details on the Callbacks API, please refer to our [Session State API Reference Guide](/develop/api-reference/caching-and-state/st.session_state#use-callbacks-to-update-session-state).

And to make all of this fast and seamless, Streamlit does some heavy lifting for you behind the scenes. A big player in this story is the `@st.cache_data` decorator, which allows developers to skip certain costly computations when their apps rerun. We'll cover caching later in this page.

## Display and style data

There are a few ways to display data (tables, arrays, data frames) in Streamlit apps. [Below](#use-magic), you will be introduced to [magic](#magic) and [st.write()](/develop/api-reference/write-magic/st.write), which can be used to write anything from text to tables. After that, let's take a look at methods designed specifically for visualizing data.

### Use magic

You can also write to your app without calling any Streamlit methods. Streamlit supports `magic commands`, `st.write()`, and `st.write_stream()`. To see this in action try this snippet:

```python
# My first app
Here's our first attempt at using data to create a table:
# st.write is a magic command that writes the data to the app
# It can be used to write text, data, Matplotlib figures, Altair charts, and more.
# For more information, refer to the documentation on magic commands.
st.write("Here's our first attempt at using data to create a table:")
st.write(pd.DataFrame({
    'first column': [1, 2, 3, 4],
    'second column': [10, 20, 30, 40]
}))
```

There are other data specific functions like `st.dataframe()`, `st.table()`, that you can also use for displaying data. Let's understand when to use these features and how to add colors and styling to your data frames.

You might be asking yourself, "why wouldn't I always use `st.write()`?" There are a few reasons:

- Magic and `st.write()` inspect the type of data that you've passed in, and then decide how to best render it in the app. Sometimes you want to draw it another way. For example, instead of drawing a dataframe as an interactive table, you may want to draw it as a static table by using `st.table(df)`.
- The second reason is that other methods return an object that can be used and modified, either by adding data to it or replacing it.
- Finally, if you use a more specific Streamlit method you can pass additional arguments to customize its behavior.

For example, let's create a data frame and change its formatting with a Pandas `Styler` object. In this example, you'll use Numpy to generate a random sample, and the `st.dataframe()` method to draw an interactive table.

This example uses Numpy to generate a random sample, but you can use Pandas DataFrames, Numpy arrays, or plain Python arrays.

```python
import streamlit as st
import numpy as np

# Generate a random sample
dataframe = np.random.randn(10, 20)
st.dataframe(dataframe)
```

Let's expand on the first example using the Pandas `Styler` object to highlight some elements in the interactive table.

```python
import streamlit as st
import numpy as np
import pandas as pd

# Generate a random sample
dataframe = pd.DataFrame(
    np.random.randn(10, 20),
    columns=('col %d' % i for i in range(20)))

st.dataframe(dataframe.style.highlight_max(axis=0))
```

Streamlit also has a method for static table generation: `st.table()`. 

```python
import streamlit as st
import numpy as np
import pandas as pd

# Generate a random sample
dataframe = pd.DataFrame(
    np.random.randn(10, 20),
    columns=('col %d' % i for i in range(20)))

st.table(dataframe)
```

### Draw charts and maps

Streamlit supports several popular data charting libraries like [Matplotlib](/develop/api-reference#chart-elements), [Altair](/develop/api-reference/charts/st.altair_chart), [deck.gl](/develop/api-reference/charts/st.graphviz_chart), and more. In this section, you'll add a bar chart, line chart, and a map to your app.

### Draw a line chart

You can easily add a line chart to your app with `st.line_chart()`. We'll generate a random sample using Numpy and then chart it.

```python
import streamlit as st
import numpy as np
import pandas as pd

# Generate a random sample
chart_data = pd.DataFrame(
    np.random.randn(20, 3),
    columns=['a', 'b', 'c'])

st.line_chart(chart_data)
```

### Plot a map

With `st.map()` you can display data points on a map. Let's use Numpy to generate some sample data and plot it on a map of San Francisco.

```python
import streamlit as st
import numpy as np
import pandas as pd

# Generate a random sample
map_data = pd.DataFrame(
    np.random.randn(1000, 2) / [50, 50] + [37.76, -122.4],
    columns=['lat', 'lon'])

st.map(map_data)
```

## Widgets

When you've got the data or model into the state that you want to explore, you can add in widgets like `st.slider()`, `st.button()`, or `st.selectbox()`. It's really straightforward — treat widgets as variables:

```python
import streamlit as st
x = st.slider('x')  # 👈 this is a widget
st.write(x, 'squared is', x * x)
```

On first run, the app above should output the text "0 squared is 0". Then every time a user interacts with a widget, Streamlit simply reruns your script from top to bottom, assigning the current state of the widget to your variable in the process.

For example, if the user moves the slider to position "10", Streamlit will rerun the code above and set "x" to "10" accordingly. So now you should see the text "10 squared is 100".

Widgets can also be accessed by key, if you choose to specify a string to use as the unique key for the widget:

```python
import streamlit as st
st.text_input('Your name', key='name')

# You can access the value at any point with:
st.session_state.name
```

Every widget with a key is automatically added to Session State. For more information about Session State, its association with widget state, and its limitations, see [Session State API Reference Guide](/develop/api-reference/caching-and-state/st.session_state).

### Use checkboxes to show/hide data

One use case for checkboxes is to hide or show a specific chart or section in an app. `st.checkbox()` takes a single argument, which is the widget label. In this sample, the checkbox is used to toggle a conditional statement.

```python
import streamlit as st
import numpy as np
import pandas as pd

if st.checkbox('Show dataframe'):
    chart_data = pd.DataFrame(
       np.random.randn(20, 3),
       columns=['a', 'b', 'c'])

    chart_data
```

### Use a selectbox for options

Use `st.selectbox` to choose from a series. You can write in the options you want, or pass through an array or data frame column.

Let's use the `df` data frame we created earlier.

```python
import streamlit as st
import pandas as pd

df = pd.DataFrame({
    'first column': [1, 2, 3, 4],
    'second column': [10, 20, 30, 40]
})

option = st.selectbox('Which number do you like best?', df['first column'])

'You selected: ', option
```

## Layout

Streamlit makes it easy to organize your widgets in a left panel sidebar with `st.sidebar`. Each element that's passed to `st.sidebar` is pinned to the left, allowing users to focus on the content in your app while still having access to UI controls.

For example, if you want to add a selectbox and a slider to a sidebar, use `st.sidebar.slider` and `st.sidebar.selectbox` instead of `st.slider` and `st.selectbox`:

```python
import streamlit as st

# Add a selectbox to the sidebar:
add_selectbox = st.sidebar.selectbox('How would you like to be contacted?', ('Email', 'Home phone', 'Mobile phone'))

# Add a slider to the sidebar:
add_slider = st.sidebar.slider('Select a range of values', 0.0, 100.0, (25.0, 75.0))

# Beyond the sidebar, Streamlit offers several other ways to control the layout of your app.
left_column, right_column = st.columns(2)

# You can use a column just like st.sidebar:
left_column.button('Press me!')

# Or even better, call Streamlit functions inside a "with" block:
with right_column:
    chosen = st.radio('Sorting hat', ('Gryffindor', 'Ravenclaw', 'Hufflepuff', 'Slytherin'))
    st.write(f"You are in {chosen} house!")
```

## Show progress

When adding long running computations to an app, you can use `st.progress()` to display status in real time.

First, let's import time. We're going to use the `time.sleep()` method to simulate a long running computation:

```python
import time
```

Now, let's create a progress bar:

```python
import streamlit as st
import time

'Starting a long computation...'
latest_iteration = st.empty()
bar = st.progress(0)

for i in range(100):
  # Update the progress bar with each iteration.
  latest_iteration.text(f'Iteration {i+1}')
  bar.progress(i + 1)
  time.sleep(0.1)
'...and now we're done!'