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Add context manager and try-finally video
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‎README.md‎

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@@ -12,6 +12,7 @@ James and his team are available for consulting, contracting, code reviews, and
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| N | Code | Video |
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|-----| --- |--- |
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| 137 | [src](videos/137_context_managers) | [The ins and outs of context managers and try-finally in Python](https://youtu.be/LBJlGwJ899Y) |
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| 136 | [src](videos/136_python_debugging) | [Python Debugging (PyCharm + VS Code)](https://youtu.be/COa-JHYuW3M) |
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| 135 | [src](videos/135_modern_logging) | [Modern Python logging](https://youtu.be/9L77QExPmI0) |
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| 134 | no src | [Why Zero times Anything is Zero](https://youtu.be/R1gxkjTJcmE) |

‎videos/137_context_managers/main.py‎

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import copy
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import decimal
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import os
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import socket
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import sys
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import tempfile
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import threading
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from contextlib import ExitStack, contextmanager, suppress
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lock = threading.Lock()
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global_state = []
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def hello():
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with open(...) as fp:
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...
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fp = open(...)
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try:
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...
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finally:
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fp.close()
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def non_cm_examples():
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fp = open("test.txt", "w")
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fp.write("Hello World")
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fp.close()
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s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
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s.settimeout(1.0) # seconds
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s.connect(("mcoding.io", 80))
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s.close()
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fp = tempfile.NamedTemporaryFile()
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fp.write(b"Hello World")
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fp.close()
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lock.acquire()
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global_state.append(42)
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lock.release()
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def cm_examples():
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with open("test.txt", "w") as fp:
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fp.write("Hello World")
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with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
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s.settimeout(1.0) # seconds
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s.connect(("mcoding.io", 80))
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with tempfile.TemporaryFile() as fp:
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fp.write(b"Hello World")
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with lock:
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global_state.append(42)
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def cm_multi_example():
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with open("test.txt") as fp:
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with open("test2.txt", "w") as fp2:
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fp2.write(fp.read())
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with open("test.txt") as fp, open("test2.txt", "w") as fp2:
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fp2.write(fp.read())
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def exit_stack_example():
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filenames = [f"test{n}.txt" for n in range(3)]
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with ExitStack() as stack:
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fps = [stack.enter_context(open(filename, "w")) for filename in filenames]
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...
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def file_example_no_cm():
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fp = open("test.txt", "w")
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fp.write("Hello World")
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fp.close()
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def file_example_no_cm_early_return(early_return_condition):
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fp = open("test.txt", "w")
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if early_return_condition:
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return
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... # many lines later
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fp.close()
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def file_example_cm():
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with open("test.txt", "w") as fp:
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fp.write("Hello World")
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def add_print_to_close(obj):
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old_close = obj.close
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def new_close():
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print("closing")
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old_close()
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obj.close = new_close
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return obj
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def file_example_cm():
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fp = open("test.txt", "w")
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fp = add_print_to_close(fp)
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print("before with")
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with fp:
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print("inside with")
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fp.write("Hello World")
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print("after with")
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def file_example_cm_with_exc():
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fp = open("test.txt", "w")
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fp = add_print_to_close(fp)
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try:
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print("before with")
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with fp:
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print("inside with")
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raise ValueError("bad")
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print("after with")
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except ValueError as exc:
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print("caught exception", exc)
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def file_example_cm_with_return():
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def inner():
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fp = open("test.txt", "w")
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fp = add_print_to_close(fp)
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print("before with")
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with fp:
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print("inside with")
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return print("returning!")
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print("after with")
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inner()
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print("after return")
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def file_example_try_finally():
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fp = open("test.txt", "w")
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try:
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fp.write("Hello World")
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finally:
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fp.close()
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def file_example_try_except_finally():
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fp = open("test.txt", "w")
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print("before try")
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try:
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raise ValueError("bad")
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except ValueError as exc:
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print("caught exception", exc)
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finally:
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print("closing")
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fp.close()
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print("after try")
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def file_example_try_finally_interrupt():
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fp = open("test.txt", "w")
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try:
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return "inside"
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finally:
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fp.close()
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return "finally"
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def try_finally_infinite_loop():
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while True:
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print("loop")
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try:
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return "inside"
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finally:
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continue
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def file_example_try_finally_lost_exc():
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fp = open("test.txt", "w")
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try:
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raise ValueError("bad")
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finally:
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fp.close()
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return # exception is now gone
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def with_statement_semantics():
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with open("test.txt", "w") as fp:
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fp.write("Hello World")
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# same as
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manager = open("test.txt", "w")
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enter = type(manager).__enter__
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exit = type(manager).__exit__
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value = enter(manager)
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hit_except = False
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try:
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fp = value
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fp.write("Hello World")
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except:
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hit_except = True
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if not exit(manager, *sys.exc_info()):
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raise
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finally:
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if not hit_except:
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exit(manager, None, None, None)
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def try_finally_vs_with_statement():
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fp = open("test.txt", "w")
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try:
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fp.write("Hello World")
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finally:
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fp.close()
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with open("test.txt", "w") as fp:
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fp.write("Hello World")
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class MyContextManager:
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def __enter__(self):
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return self
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def __exit__(self, exc_type, exc_val, exc_tb):
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...
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def my_cm_example():
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with MyContextManager() as cm:
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raise ValueError
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def suppress_example():
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# remove it if it exists,
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# otherwise I don't care
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with suppress(OSError):
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os.remove("test.txt")
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def _api_begin(arg):
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print(f"api begin: {arg}")
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return {"status": "ok"}
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def _api_end():
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print("api end")
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def api_begin(arg):
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val = _api_begin(arg)
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return _ExternalAPIContextManager(val)
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class _ExternalAPIContextManager:
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def __init__(self, val):
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self.val = val
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def __enter__(self):
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return self.val
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def __exit__(self, exc_type, exc_val, exc_tb):
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_api_end()
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def external_api_example():
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val = _api_begin("arg")
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print(f"use val: {val}")
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_api_end()
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# instead
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with api_begin("arg") as val:
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print(f"use val: {val}")
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@contextmanager
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def api_begin2(arg):
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val = _api_begin(arg)
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yield val
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_api_end()
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@contextmanager
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def api_begin3(arg):
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val = _api_begin(arg)
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try:
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yield val
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finally:
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_api_end()
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class ActualContextManager:
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def __enter__(self):
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global global_state
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self.saved = global_state
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global_state = copy.deepcopy(global_state)
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# mutate copy if desired
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return global_state
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def __exit__(self, exc_type, exc_val, exc_tb):
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global global_state
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global_state = self.saved
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self.saved = None
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def pi():
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# https://docs.python.org/3/library/decimal.html#module-decimal
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decimal.getcontext().prec += 2
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three = decimal.Decimal(3)
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lasts, t, s, n, na, d, da = 0, three, 3, 1, 0, 0, 24
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while s != lasts:
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lasts = s
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n, na = n + na, na + 8
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d, da = d + da, da + 32
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t = (t * n) / d
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s += t
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decimal.getcontext().prec -= 2
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return +s
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def decimal_example():
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print(pi())
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with decimal.localcontext(prec=100):
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print(pi())
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def nice_exit_example():
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try:
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sys.exit(1)
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finally:
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print("printing on the way out...")
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def not_nice_exit_example():
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try:
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os._exit(1)
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finally:
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print("this does not print")
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class UnluckyContextManager:
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def __enter__(self):
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lock.acquire()
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def __exit__(self, exc_type, exc_val, exc_tb):
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lock.release()
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def unlucky_context_manager_example():
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try:
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while True: # interrupt me!
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with lock:
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pass
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finally:
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print(f"lock released on exit? {not lock.locked()}")
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def main():
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# cm_examples()
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# non_cm_examples()
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# cm_multi_example()
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# file_example_cm()
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# file_example_cm_with_exc()
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# file_example_cm_with_return()
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# file_example_try_finally_lost_exc()
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# try_finally_infinite_loop()
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# with_statement_semantics()
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# try_finally_vs_with_statement()
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# my_cm_example()
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# suppress_example()
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# external_api_example()
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# decimal_example()
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# nice_exit_example()
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# not_nice_exit_example()
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unlucky_context_manager_example()
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if __name__ == "__main__":
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main()

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