1
0
mirror of https://github.com/satwikkansal/wtfpython synced 2024-11-21 18:54:20 +01:00

Add examples:

* The disappearing variable to outer scope
* Return in both `try` and `finally` clauses
This commit is contained in:
Satwik Kansal 2017-08-28 21:37:06 +05:30
parent f45b8abeeb
commit 4c922b3280

159
wtfpy.md
View File

@ -409,10 +409,167 @@ def another_func():
UnboundLocalError: local variable 'a' referenced before assignment
```
**Explanation:**
### Explanation
When you make an assignment to a variable in a scope, it becomes local to that scope. So `a` becomes local to the scope of `another_func` but it has not been initialized previously in the same scope which throws an error. Read [this](http://sebastianraschka.com/Articles/2014_python_scope_and_namespaces.html) short but awesome guide to learn more about how namespaces and scope resolution works in Python.
## The disappearing variable from outer scope
```py
e = 7
try:
raise Exception()
except Exception as e:
pass
```
**Output (Python 2.x):**
```py
>>> print(e)
# prints nothing
```
**Output (Python 3.x):**
```py
>>> print(e)
NameError: name 'e' is not defined
```
### Explanation
* Source: https://docs.python.org/3/reference/compound_stmts.html#except
When an exception has been assigned using as target, it is cleared at the end of the except clause. This is as if
```py
except E as N:
foo
```
was translated to
```py
except E as N:
try:
foo
finally:
del N
```
This means the exception must be assigned to a different name to be able to refer to it after the except clause. Exceptions are cleared because with the traceback attached to them, they form a reference cycle with the stack frame, keeping all locals in that frame alive until the next garbage collection occurs.
* The clauses are not scoped in Python. Everything in the example is present in same scope and the variable `e` got removed due to the execution of the `except` clause. The same is not the case with functions which have their separate inner-scopes. The example below illustrates this:
```py
def f(x):
del(x)
print(x)
x = 5
y = [5, 4, 3]
```
**Output:**
```py
>>>f(x)
UnboundLocalError: local variable 'x' referenced before assignment
>>>f(y)
UnboundLocalError: local variable 'x' referenced before assignment
>>> x
5
>>> y
[5, 4, 3]
```
* In Python 2.x the variable name `e` gets assigned to `Exception()` instance, so when you try to print, it prints nothing.
**Output (Python 2.x):**
```py
>>> e
Exception()
>>> print e
# Nothing is printed!
```
## Return in both `try` and `finally` clauses
```py
def some_func():
try:
return 'from_try'
finally:
return 'from_finally'
```
**Output:**
```py
>>> some_func()
'from_finally'
```
### Explanation
When a `return`, `break` or `continue` statement is executed in the `try` suite of a "try…finally" statement, the `finally` clause is also executed on the way out. The return value of a function is determined by the last `return` statement executed. Since the `finally` clause always executes, a `return` statement executed in the `finally` clause will always be the last one executed.
## When True is actually False
## The GIL messes it up (Multithreading vs Mutliprogramming example)
## Take care of the operator precedence buddy! (located inside GIL thread)
```py
>>> True is False == False
False
>>> False is False is False
True
```
## Implicit conversion can hurt sometimes
```py
>>> True + 1
2
```
## a += b doesn't behave the same way as a = a+b
```
>>> a=[1,2,3,4]
>>> b=a
>>> a=a+[5,6,7,8]
>>> a
[1, 2, 3, 4, 5, 6, 7, 8]
>>> b
[1, 2, 3, 4]
>>> a=[1,2,3,4]
>>> b=a
>>> a+=[5,6,7,8]
>>> a
[1, 2, 3, 4, 5, 6, 7, 8]
>>> b
[1, 2, 3, 4, 5, 6, 7, 8]
```
## That "is" on the same non-static method of the class instance returns False. It was the first and the last time I tried prettify my code with "is".
## Some title
```
x = {0: None}
for i in x:
del x[i]
x[i+1] = None
print i
```
## Minor ones
- join() is a string operation instead of list operation. (sort of counterintuitive)
- No multicore support yet
# Contributing
All patches are Welcome! Filing an issue first before submitting a patch will be appreciated :)