Structure
- Python course
- Expert
- Lesson 23/47
- 5 min
Learn C structs with ctypes.Structure.
What is a Structure?
ctypes.Structure represents C structs in Python. You specify field names and types with the _fields_ list.
For example class Point(Structure): _fields_ = [('x', c_int), ('y', c_int)] creates a struct with x and y fields. You instantiate with p = Point(3, 4) and access with p.x. This lets you use C library structs directly in Python, which is very useful for reading binary data.
Field access
After creating a Structure instance you access fields with a dot: p.x, p.y. Both reading and writing work.
The field order matches the _fields_ list. Point(3, 4) sets the first field to 3 and the second to 4. Nested structures are supported: a field's type can be another Structure. This lets you easily model complex C data structures in Python.
Practice Define a point with Structure
Import the module with import ctypes. Write a Point(ctypes.Structure) class; define _fields_ = [('x', ctypes.c_int), ('y', ctypes.c_int)]. Create an instance with Point(3, 4) and print p.x and p.y.
Starter code
# write the Point struct
# create an instance and printSample solution
import ctypes
class Point(ctypes.Structure):
_fields_ = [("x", ctypes.c_int), ("y", ctypes.c_int)]
p = Point(3, 4)
print(p.x, p.y)Expected output
3 4
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which attribute does Structure use?
class Point(ctypes.Structure):
_fields_ = [('x', c_int)]- __init__
- __dict__
- __slots__
- _fields_ Correct answer
Explanation: answer: _fields_. This list defines field names and types.
2 Match Structure concepts
| Term | Meaning |
|---|---|
Structure |
C struct base |
_fields_ |
Field list |
Point(3,4) |
Creating an instance |
p.x |
Field access |
3 Complete the Structure definition
class Point(ctypes.____):
_fields_ = [('x', c_int)]class Point(ctypes.Structure):
_fields_ = [('x', c_int)]Explanation: A struct derives from ctypes.Structure.