The PicklingError: Can't pickle local object occurs when attempting to serialize (pickle) an object that is defined within a local scope, such as inside a function, lambda, or class method. This is common when using multiprocessing in Python.
1. Understanding the Error
Pickle is Python’s module for serializing and deserializing objects. However, it cannot serialize objects defined in:
- Local functions
- Lambda functions
- Inner classes
- Instance methods of classes
- Objects requiring external resources (e.g., file handles, sockets, or threads)
2. Common Scenarios and Fixes
A. Function Defined Inside Another Function
Example (Causing the Error)
import pickle
def outer():
def inner_function(x):
return x * x
pickle.dumps(inner_function) # Causes PicklingError
outer()
Fix: Define the function at the top level
import pickle
def inner_function(x):
return x * x
pickle.dumps(inner_function) # No error
B. Using multiprocessing with a Locally Defined Function
Example (Causing the Error)
import multiprocessing
def process_data():
def task(x):
return x * x
with multiprocessing.Pool(2) as pool:
pool.map(task, [1, 2, 3]) # PicklingError occurs
process_data()
Fix: Move task() outside
import multiprocessing
def task(x):
return x * x
def process_data():
with multiprocessing.Pool(2) as pool:
pool.map(task, [1, 2, 3])
process_data()
C. Using lambda in Pickle
Example (Causing the Error)
import pickle
func = lambda x: x + 1
pickle.dumps(func) # PicklingError
Fix: Use a named function
def add_one(x):
return x + 1
pickle.dumps(add_one) # No error
D. Pickling an Instance Method
Example (Causing the Error)
import pickle
class MyClass:
def method(self):
return "Hello"
obj = MyClass()
pickle.dumps(obj.method) # PicklingError
Fix: Use dill or store only the object, not the method
import dill # Alternative to pickle that supports methods
pickle.dumps(obj) # No error (pickling the object)
dill.dumps(obj.method) # No error (dill can handle methods)
E. Inner Class Pickling Issue
Example (Causing the Error)
import pickle
class Outer:
class Inner:
pass
pickle.dumps(Outer.Inner()) # PicklingError
Fix: Define the class at the top level
class Inner:
pass
pickle.dumps(Inner()) # No error
3. Alternative Solutions
A. Use dill Instead of pickle
Dill extends pickle and supports local functions and lambdas.
import dill
def local_function(x):
return x * 2
print(dill.dumps(local_function)) # Works
B. Use pathos.multiprocessing Instead of multiprocessing
If using multiprocessing, replace it with pathos.multiprocessing, which works with local functions.
from pathos.multiprocessing import ProcessingPool as Pool
def local_function(x):
return x * 2
with Pool(2) as pool:
print(pool.map(local_function, [1, 2, 3])) # Works
4. Summary
| Cause | Fix |
|---|---|
| Local function inside another function | Define it at the top level |
Using lambda | Replace with a named function |
| Using an instance method | Store only the object, or use dill |
| Pickling an inner class | Move the class to the top level |
Using multiprocessing with local functions | Use pathos.multiprocessing or move the function |