ch07

ch07/context/decimal.prec.py

@@ -0,0 +1,46 @@
+# context/decimal.prec.py
+from decimal import Context, Decimal, getcontext, setcontext
+
+one = Decimal("1")
+three = Decimal("3")
+
+orig_ctx = getcontext()
+ctx = Context(prec=5)
+setcontext(ctx)
+print(ctx)
+print(one / three)
+setcontext(orig_ctx)
+print(one / three)
+
+
+"""
+$ python context/decimal.prec.py
+Context(prec=5, rounding=ROUND_HALF_EVEN, Emin=-999999,
+Emax=999999, capitals=1, clamp=0, flags=[],
+traps=[InvalidOperation, DivisionByZero, Overflow])
+0.33333
+0.3333333333333333333333333333
+"""
+
+
+orig_ctx = getcontext()
+ctx = Context(prec=5)
+setcontext(ctx)
+try:
+    print(ctx)
+    print(one / three)
+finally:
+    setcontext(orig_ctx)
+print(one / three)
+
+
+from decimal import localcontext
+
+with localcontext(Context(prec=5)) as ctx:
+    print(ctx)
+    print(one / three)
+print(one / three)
+
+
+with localcontext(Context(prec=5)), open("out.txt", "w") as out_f:
+    out_f.write(f"{one} / {three} = {one / three}\n")

ch07/context/generator.py

@@ -0,0 +1,38 @@
+# context/generator.py
+from contextlib import contextmanager
+
+@contextmanager
+def my_context_manager():
+    print("Entering 'with' context")
+    val = object()
+    print(id(val))
+    try:
+        yield val
+    except Exception as e:
+        print(f"{type(e)=} {e=} {e.__traceback__=}")
+    finally:
+        print("Exiting 'with' context")
+
+
+print("About to enter 'with' context")
+with my_context_manager() as val:
+    print("Inside 'with' context")
+    print(id(val))
+    raise Exception("Exception inside 'with' context")
+    print("This line will never be reached")
+
+print("After 'with' context")
+
+
+"""
+$ python context/generator.py
+About to enter 'with' context
+Entering 'with' context
+139768531985040
+Inside 'with' context
+139768531985040
+type(e)=<class 'Exception'> e=Exception("Exception inside 'with'
+ context") e.__traceback__=<traceback object at 0x7f1e65a42800>
+Exiting 'with' context
+After 'with' context
+"""

ch07/context/manager.class.py

@@ -0,0 +1,40 @@
+# context/manager.class.py
+class MyContextManager:
+    def __init__(self):
+        print("MyContextManager init", id(self))
+
+    def __enter__(self):
+        print("Entering 'with' context")
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        print(f"{exc_type=} {exc_val=} {exc_tb=}")
+        print("Exiting 'with' context")
+        return True
+
+
+ctx_mgr = MyContextManager()
+
+print("About to enter 'with' context")
+
+with ctx_mgr as mgr:
+    print("Inside 'with' context")
+    print(id(mgr))
+    raise Exception("Exception inside 'with' context")
+    print("This line will never be reached")
+
+print("After 'with' context")
+
+
+"""
+$ python context/manager.class.py
+MyContextManager init 140340228792272
+About to enter 'with' context
+Entering 'with' context
+Inside 'with' context
+140340228792272
+exc_type=<class 'Exception'> exc_val=Exception("Exception inside
+ 'with' context") exc_tb=<traceback object at 0x7fa3817c5340>
+Exiting 'with' context
+After 'with' context
+"""

ch07/exceptions/first.example.py

@@ -0,0 +1,31 @@
+# exceptions/first.example.py
+# This is not a valid Python module - Don't run it.
+# a few examples of exceptions
+
+>>> gen = (n for n in range(2))
+>>> next(gen)
+0
+>>> next(gen)
+1
+>>> next(gen)
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+StopIteration
+>>> print(undefined_name)
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+NameError: name 'undefined_name' is not defined
+>>> mylist = [1, 2, 3]
+>>> mylist[5]
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+IndexError: list index out of range
+>>> mydict = {'a': 'A', 'b': 'B'}
+>>> mydict['c']
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+KeyError: 'c'
+>>> 1 / 0
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+ZeroDivisionError: division by zero

ch07/exceptions/for.loop.py

@@ -0,0 +1,25 @@
+# exceptions/for.loop.py
+n = 100
+found = False
+for a in range(n):
+    if found: break
+    for b in range(n):
+        if found: break
+        for c in range(n):
+            if 42 * a + 17 * b + c == 5096:
+                found = True
+                print(a, b, c)  # 79 99 95
+
+
+class ExitLoopException(Exception):
+    pass
+
+try:
+    n = 100
+    for a in range(n):
+        for b in range(n):
+            for c in range(n):
+                if 42 * a + 17 * b + c == 5096:
+                    raise ExitLoopException(a, b, c)
+except ExitLoopException as ele:
+    print(ele.args)  # (79, 99, 95)

ch07/exceptions/multiple.py

@@ -0,0 +1,15 @@
+# exceptions/multiple.py
+values = (1, 2)  # try (1, 0) and ('one', 2)
+
+try:
+    q, r = divmod(*values)
+except (ZeroDivisionError, TypeError) as e:
+    print(type(e), e)
+
+
+try:
+    q, r = divmod(*values)
+except ZeroDivisionError:
+    print("You tried to divide by zero!")
+except TypeError as e:
+    print(e)

ch07/exceptions/raising.py

@@ -0,0 +1,6 @@
+# exceptions/raising.py
+# This is not a valid Python module - Don't run it.
+>>> raise NotImplementedError("I'm afraid I can't do that")
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+NotImplementedError: I'm afraid I can't do that

ch07/exceptions/replace.py

@@ -0,0 +1,34 @@
+# exceptions/replace.py
+# This is not a valid Python module - Don't run it.
+>>> class NotFoundError(Exception):
+...     pass
+...
+>>> vowels = {'a': 1, 'e': 5, 'i': 9, 'o': 15, 'u': 21}
+>>> try:
+...     pos = vowels['y']
+... except KeyError as e:
+...     raise NotFoundError(*e.args)
+...
+Traceback (most recent call last):
+  File "<stdin>", line 2, in <module>
+KeyError: 'y'
+
+During handling of the above exception, another exception occurred:
+
+Traceback (most recent call last):
+  File "<stdin>", line 4, in <module>
+__main__.NotFoundError: y
+>>> try:
+...     pos = vowels['y']
+... except KeyError as e:
+...     raise NotFoundError(*e.args) from e
+...
+Traceback (most recent call last):
+  File "<stdin>", line 2, in <module>
+KeyError: 'y'
+
+The above exception was the direct cause of the following exception:
+
+Traceback (most recent call last):
+  File "<stdin>", line 4, in <module>
+__main__.NotFoundError: y

ch07/exceptions/trace.back.py

@@ -0,0 +1,25 @@
+# exceptions/trace.back.py
+def squareroot(number):
+    if number < 0:
+        raise ValueError("No negative numbers please")
+    return number ** .5
+
+def quadratic(a, b, c):
+    d = b ** 2 - 4 * a * c
+    return ((-b - squareroot(d)) / (2 * a),
+            (-b + squareroot(d)) / (2 * a))
+
+quadratic(1, 0, 1)  # x**2 + 1 == 0
+
+
+"""
+$ python exceptions/trace.back.py
+Traceback (most recent call last):
+  File "exceptions/trace.back.py", line 12, in <module>
+    quadratic(1, 0, 1)  # x**2 + 1 == 0
+  File "exceptions/trace.back.py", line 9, in quadratic
+    return ((-b - squareroot(d)) / (2 * a),
+  File "exceptions/trace.back.py", line 4, in squareroot
+    raise ValueError("No negative numbers please")
+ValueError: No negative numbers please
+"""

ch07/exceptions/try.syntax.py

@@ -0,0 +1,30 @@
+# exceptions/try.syntax.py
+
+def try_syntax(numerator, denominator):
+    try:
+        print(f'In the try block: {numerator}/{denominator}')
+        result = numerator / denominator
+    except ZeroDivisionError as zde:
+        print(zde)
+    else:
+        print('The result is:', result)
+        return result
+    finally:
+        print('Exiting')
+
+print(try_syntax(12, 4))
+print(try_syntax(11, 0))
+
+
+"""
+$ python try.syntax.py
+In the try block: 12/4    # try
+The result is: 3.0        # else
+Exiting                   # finally
+3.0                       # return within else
+
+In the try block: 11/0    # try
+division by zero          # except
+Exiting                   # finally
+None                      # implicit return end of function
+"""

ch07/exceptions/unhandled.py

@@ -0,0 +1,11 @@
+# exceptions/unhandled.py
+1 + "one"
+print("This line will never be reached")
+
+"""
+$ python exceptions/unhandled.py
+Traceback (most recent call last):
+  File "exceptions/unhandled.py", line 3, in <module>
+    1 + "one"
+TypeError: unsupported operand type(s) for +: 'int' and 'str'
+"""