Possible Windows-only error on save() to PNG

Getting an error using Altair saver when I am trying to save a chart to a PNG in Jupyter Notebook: ValueError: Unsupported format: 'png'

Answer here on Stack Overflow suggested filing a bug report.

System info:

Windows 10 conda 4.8.2 Python 3.8.3 altair 4.1.0 py_1 conda-forge altair_saver 0.1.0 py_0 conda-forge vega 3.4.0 py38h32f6830_0 conda-forge selenium 3.141.0 py38h9de7a3e_1001 conda-forge

Example Code:

import pandas as pd
import altair as alt
from altair_saver import save
alt.renderers.enable('default'); # if in jupyter, ; to suppress output
alt.renderers.enable('altair_saver', fmts=['vega-lite', 'png']);

mytaskbars = pd.DataFrame([
    {"task": "Task1a", "start": '2020-06-01', "end": '2020-09-30', "color": 'royalblue'},
    {"task": "Task1b", "start": '2020-06-01', "end": '2021-03-31', "color": 'deepskyblue'},
    {"task": "Task2", "start": '2020-06-01', "end": '2021-03-31', "color": 'red'},
    ])
    
mytaskbars["start"] = pd.to_datetime(mytaskbars["start"])
mytaskbars["end"] = pd.to_datetime(mytaskbars["end"])
    
    
chart = alt.Chart(mytaskbars).mark_bar(opacity=0.7).encode(
    x=alt.X('start', axis=alt.Axis(title='Date', labelAngle=-45, format = ("%b %Y"))),
     x2 = 'end',
    y=alt.Y('task', axis=alt.Axis(title=None)),
    color = alt.Color('color:N', scale = None)
    )
    
save(chart, "chart_202006.png")
chart

Error Message:

ValueError                                Traceback (most recent call last)
<ipython-input-3-13a284c2aca9> in <module>
     19     )
     20 
---> 21 save(chart, "chart_202006.png")
     22 chart

~\anaconda3\envs\geospat_env\lib\site-packages\altair_saver\_core.py in save(chart, fp, fmt, mode, method, **kwargs)
     60     """
     61     if method is None:
---> 62         Saver = _get_saver_for_format(fmt=fmt, fp=fp)
     63     elif isinstance(method, type):
     64         Saver = method

~\anaconda3\envs\geospat_env\lib\site-packages\altair_saver\_core.py in _get_saver_for_format(fmt, fp)
     28         if fmt in s.valid_formats and s.enabled():
     29             return s
---> 30     raise ValueError(f"Unsupported format: {fmt!r}")
     31 
     32 

ValueError: Unsupported format: 'png'

[dkruszew]

I encountered the same issue while running the altair_saver.save() function on my Windows 10 machine: i.e this error: No enabled saver found that supports format='png'.

Investigating the error led me to the below code within _core.py in the altair_saver package, which indicated that the error statement I was receiving was likely due to the below if statement failing for each saver method:

for s in _SAVER_METHODS.values():
    if s.enabled() and fmt in s.valid_formats[mode]:
        return s
raise ValueError(f"No enabled saver found that supports format={fmt!r}")

I originally thought that the above if statements were failing because of the file format png in s.valid_formats[mode]. Upon investigation I discovered the png string was fine, and the issue appeared to exist within s.enabled(). I did some further digging where I played with the method and webdriver arguments in the save function. When respectively using method=selenium with webdriver=chrome I received an error that told me my chromedriver.exe file was out of date for my chrome browser. I re-downloaded from here and replaced the file in C:\Windows. After doing so the save function worked seamlessly.

[jhermann]

You need to find out the difference in your shell env vs. the systemd one – starting with the fact you run this as root. If you try the ExecStart command in a root shell that might enlighten you as to a cause.

Or look into systemd user services and run as vtn-bot.