# A nonexistent object was used in an Input of a Dash callback

**URL:** <https://community.plotly.com/t/a-nonexistent-object-was-used-in-an-input-of-a-dash-callback/92309>\
**Category:** Dash Python\
**Tags:** question\
**Created:** [May 15, 2025, 1:51pm UTC](https://community.plotly.com/t/a-nonexistent-object-was-used-in-an-input-of-a-dash-callback/92309 "2025-05-15T13:51:32Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![JulieJacob](https://avatars.discourse-cdn.com/v4/letter/j/67e7ee/32.png) [@JulieJacob](https://community.plotly.com/u/JulieJacob)\
**Post date:** [May 15, 2025, 1:51pm UTC](https://community.plotly.com/t/a-nonexistent-object-was-used-in-an-input-of-a-dash-callback/92309/1 "2025-05-15T13:51:32Z")

</div>

I am building an image-processing app in dash, where the user uploads an image, selects a smoothing filter and then chooses between k-means clustering and thresholding to identify the foreground object from the background.

As shown below, I have a callback with a `filters-drop down` (which works fine as both filters take the same type of input - in this case the `kernel-height-slider` and the `kernel-width-slider`.

```lang-py
@app.callback(
    Output('smoothed-img','src'), 
    [
        Input('filters-dropdown', 'value'),
        Input('kernel-height-slider', 'value'),
        Input('kernel-width-slider', 'value'),
        Input('detectors-radio', 'value'),
        Input('otsu-min-threshold', 'value'),
        Input('otsu-max-threshold', 'value'),     
        Input('kmeans-kvalue-input', 'value'),
        Input('kmeans-niter-input', 'value'),   
        Input('kmeans-accuracy-input', 'value'),
    ]
)
def update_overlay_base64(selected_filter, kernel_height, kernel_width, 
                          selected_detector, otsu_min_thr, otsu_max_thr, 
                          kmeans_k, kmeans_niter, kmeans_accuracy):
    if not selected_filter:
        return None
 
    filtered_img = filters[selected_filter](preprocessed_img, 
    kernel_height, kernel_width)

    if selected_detector == "otsu" and otsu_min_thr is not None and 
    otsu_max_thr is not None:
        processed_img = otsu_thresholding(filtered_img, otsu_min_thr, 
    otsu_max_thr)[1]
        
    elif selected_detector == "kmeans" and None not in (kmeans_k, 
    kmeans_niter, kmeans_accuracy):
        processed_img = kmeans_clustering(filtered_img, kmeans_k, 
    kmeans_niter, kmeans_accuracy)

    edge_img = cv.Canny(processed_img, 10, 150)
    edge_img = np.array(edge_img, dtype='float64')
    edge_img[edge_img==0] = np.nan
    rows, cols = np.where(edge_img == 255)
    min_row, max_row = rows.min(), rows.max()
    min_col, max_col = cols.min(), cols.max()

    # Plot with matplotlib
    fig, ax = plt.subplots()
    ax.imshow(processed_img, cmap='Spectral_r')
    ax.imshow(edge_img, cmap='autumn', alpha=0.5)
    plt.gca().add_patch(
        plt.Rectangle((min_col-1, min_row-1), 
                        max_col - min_col + 2, 
                        max_row - min_row + 2, 
                        edgecolor='cyan', 
                        facecolor='none', 
                        linewidth=1)
    )    
    ax.axis('off')    

    # Save plot to buffer
    buf = io.BytesIO()
    plt.savefig(buf, format='png', bbox_inches='tight', pad_inches=0)
    plt.close(fig)
    buf.seek(0)
 
    # Encode to base64
    encoded = base64.b64encode(buf.read()).decode()
    return f"data:image/png;base64,{encoded}"

```

However, when choosing from the `detectors-dropdown`, depending on which one is selected the inputs for the image processing function will either be `otsu-min-threshold` and `otsu-max-threshold` (if otsu is selected) OR `kmeans-kvalue-input`, `kmeans-niter-input` and `kmeans-accuracy-input` (if kmeans is selected).

The issue is that the callback needs all the `Inputs` to work properly and throws errors as inevitably, one set of input is not available when the other option is selected. Is there a way to work around this? Can I do conditional callbacks or something similar?

Thank you!

---

<div class="post-metadata">

**Author:** ![jinnyzor](https://sea2.discourse-cdn.com/flex024/user_avatar/community.plotly.com/jinnyzor/32/21194_2.png) [@jinnyzor](https://community.plotly.com/u/jinnyzor)\
**Post date:** [May 15, 2025, 2:01pm UTC](https://community.plotly.com/t/a-nonexistent-object-was-used-in-an-input-of-a-dash-callback/92309/2 "2025-05-15T14:01:17Z")

</div>

Hello @JulieJacob,

Welcome to the community!

There will be `allow_optional` in the future for State and Inputs, this will help with this issue. Not sure when the next release of Dash will be.

There are a couple of things you can do currently:  
1 - use pattern-matching callbacks instead, these allow for there to non-existant values  
2 - split the callbacks

---

<div class="post-metadata">

**Author:** ![JulieJacob](https://avatars.discourse-cdn.com/v4/letter/j/67e7ee/32.png) [@JulieJacob](https://community.plotly.com/u/JulieJacob)\
**Post date:** [May 17, 2025, 2:02pm UTC](https://community.plotly.com/t/a-nonexistent-object-was-used-in-an-input-of-a-dash-callback/92309/3 "2025-05-17T14:02:21Z")

</div>

Hi @jinnyzor thank you so much!