<img src="https://media.daily.dev/image/upload/s--fQCe6rUw--/f_auto/v1753805369/ugc/content_754a5e6c-d454-400f-a6ae-432df79cbfcb?_a=BAMClqZW0" alt="carbon">
Recently, I came across an interesting issue in a Python code that was generating files with a unique name using this pattern:
<pre><code>𝘪𝘮𝘱𝘰𝘳𝘵 𝘶𝘶𝘪𝘥
𝘥𝘦𝘧 𝘤𝘳𝘦𝘢𝘵𝘦_𝘧𝘪𝘭𝘦(𝘧𝘪𝘭𝘦𝘯𝘢𝘮𝘦=𝘶𝘶𝘪𝘥.𝘶𝘶𝘪𝘥4()):
 𝘱𝘳𝘪𝘯𝘵(𝘧&quot;𝘊𝘳𝘦𝘢𝘵𝘪𝘯𝘨 𝘧𝘪𝘭𝘦: {𝘧𝘪𝘭𝘦𝘯𝘢𝘮𝘦}&quot;)
 # 𝘚𝘪𝘮𝘶𝘭𝘢𝘵𝘦 𝘧𝘪𝘭𝘦 𝘤𝘳𝘦𝘢𝘵𝘪𝘰𝘯/𝘥𝘦𝘭𝘦𝘵𝘪𝘰𝘯 𝘭𝘰𝘨𝘪𝘤 𝘩𝘦𝘳𝘦
</code></pre>
On the surface, it looked fine—𝘂𝘂𝗶𝗱.𝘂𝘂𝗶𝗱𝟰() should generate a new UUID each time, right?
But I kept getting “𝗙𝗶𝗹𝗲 𝗡𝗼𝘁 𝗙𝗼𝘂𝗻𝗱” errors when triggering it multiple times concurrently. After debugging, I realized the same UUID was being used every time, and the file was being deleted by one thread while another still expected it.
𝗧𝗵𝗲 𝗰𝗮𝘁𝗰𝗵?
In Python, default parameters are evaluated only once, when the function is defined, not every time it’s called. So, 𝘂𝘂𝗶𝗱.𝘂𝘂𝗶𝗱𝟰() was only called once, and the same UUID was reused.
In 𝗝𝗮𝘃𝗮𝗦𝗰𝗿𝗶𝗽𝘁, default parameters like 𝗠𝗮𝘁𝗵.𝗿𝗮𝗻𝗱𝗼𝗺() are evaluated each time the function is called, unlike Python.
#PythonTips #Debugging #UUID #DefaultParameters #JavaScriptVsPython #DevLife #SoftwareEngineering #CodeQuality #python

How default parameter works in Python?