The Secret Life of Python - The Uncopyable (deepcopy)
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The Secret Life of Python - The Uncopyable (deepcopy)
When deepcopy fails: why some Python objects can't be cloned.
#Python #Coding #DeepCopy #BuilderPattern
🎧 Audio Edition: Prefer to listen? Check out the expanded AI podcast version of this deep dive on YouTube.
📺 Video Edition: Prefer to watch? Check out the 7-minute visual explainer on YouTube.
Timothy was feeling like a master of the universe. After learning about deepcopy, he felt invincible. No ghost could haunt his data again.
"Margaret, I’ve solved everything," Timothy beamed. "I’m writing a script to back up my tournament database. I just deep-copied the entire Connection object so I have a 'safe' copy to experiment with."
Margaret froze mid-sip of her tea. "You did what, Timothy?"
"I deep-copied the database connection," Timothy repeated, his smile faltering. "But Python just... exploded."
He showed her the screen. Instead of a clean copy, he had a wall of red text ending in a terrifying TypeError: cannot pickle '_thread.lock' object.
"I thought deepcopy was the ultimate clone machine," Timothy said. "Why does it mention 'pickling,' and why did it quit on me?"
The Physical Link
Margaret sighed, but there was a twinkle of pride in her eye. "You’ve found the edge of the map, Timothy. You tried to photocopy a physical telephone line."
"As for the 'pickle' error," she added, "that’s just Python’s internal way of saying it tried to turn your object into a stream of data to move it. deepcopy uses pickling under the hood. But it hit a brick wall."
She pointed to the router on the wall. "A database connection, a file handle, or a network socket isn't just data. It’s a live link to something outside of Python. It's a physical relationship with the operating system."
"Imagine you have a list of your friends' names," Margaret continued. "You can photocopy that list all day. But if the list includes a live phone call to your friend, can you photocopy the call?"
"No," Timothy realized. "I’d have the number, but I wouldn't have a second person on a second line."
The "Zombie" Corruption
"But what if it didn't crash?" Timothy asked. "Wouldn't that be better?"
"Actually, no," Margaret warned. "That’s where it gets dangerous. If Python 'silently' copied the data but couldn't copy the connection, you’d end up with a Corrupted State. You’d have a 'zombie' copy—it looks like a connection, but it’s a hollow shell. If you tried to send data through it, your program might hang forever or crash the whole database."
The Workaround: The Builder Pattern
"So if I can't copy it, how do I get a second one?"
"You don't copy the object," Margaret explained. "You copy the Instructions. If you want a second database connection, you don't clone the first one; you just open a brand new one using the same configuration."
She showed him the "Builder" approach:
# The WRONG way (The Crash)
# backup_conn = copy.deepcopy(original_conn)
# The RIGHT way (The Builder)
def create_connection(config):
# This creates a fresh twin from scratch
return database.connect(config)
db_config = {"host": "localhost", "user": "admin"}
conn1 = create_connection(db_config)
conn2 = create_connection(db_config)
"You copy the config (the map)," Margaret said, "not the connection (the journey)."
Margaret’s Cheat Sheet: The Limits of Magic
Margaret added a high-level warning to Timothy's notebook:
- Is it pure Data? (Lists, dicts, custom info classes) →
deepcopyworks ✅. - Is it a Resource? (Files, DB connections, Sockets, Thread locks) →
deepcopyfails ❌. - The Solution: Use a Builder or Factory function to create a fresh twin from the original input.
- Advanced Note: Objects can technically define their own
__deepcopy__method to handle this, but for system resources, it's almost always safer to just start fresh.
Timothy looked at the red error text. "So even the 'ultimate' tool has a limit."
"The best engineers," Margaret said, "don't look for a tool that does everything. They look for the right tool for the specific material they are working with."
In the next episode, Margaret and Timothy will face "The Default Trap"—where Timothy learns that giving a function a default list is like sharing a toothbrush with a stranger.
Aaron Rose is a software engineer and technology writer at tech-reader.blog. For explainer videos and podcasts, check out Tech-Reader YouTube channel.
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