Add more examples.

* Needles in Haystack bugs
* Chained comparisons
This commit is contained in:
Satwik Kansal 2017-08-28 23:03:10 +05:30
parent 4c922b3280
commit 0d0e6d1052
1 changed files with 77 additions and 2 deletions

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@ -514,17 +514,57 @@ When a `return`, `break` or `continue` statement is executed in the `try` suite
## When True is actually False
```py
True == False
if True == False:
print("I've lost faith in truth!")
```
**Output:**
```
I've lost faith in truth!
```
### Explanation
Initially, Python used to have no `bool` type (people used 0 for false and non-zero value like 1 for true). Then they added `True`, `False`, and a `bool` type, but, for backwards compatibility, they couldn't make `True` and `False` constants- they just were built-in variables.
Python 3 was backwards-incompatible, so it was now finally possible to fix that, and so this example wont't work with Python 3.x.
## The GIL messes it up (Multithreading vs Mutliprogramming example)
## Take care of the operator precedence buddy! (located inside GIL thread)
## Be careful with chained comparisons
```py
>>> True is False == False
False
>>> False is False is False
True
>>> 1 > 0 < 1
True
>>> (1 > 0) < 1
False
>>> 1 > (0 < 1)
False
```
### Explanation
As per https://docs.python.org/2/reference/expressions.html#not-in
> Formally, if a, b, c, ..., y, z are expressions and op1, op2, ..., opN are comparison operators, then a op1 b op2 c ... y opN z is equivalent to a op1 b and b op2 c and ... y opN z, except that each expression is evaluated at most once.
* `False is False is False` is equivalent to `(False is False) and (False is False)`
* `True is False == False` is equivalent to `True is False and False == False` and since the first part of the statement (`True is False`) evaluates to `False`, the overall expression evaluates to `False`.
* `1 > 0 < 1` is equivalent to `1 > 0 and 0 < 1` which evaluates to `True`.
* The expression `(1 > 0) < 1` is equivalent to `True < 1` and
```py
>>> int(True)
1
```
So, `1 < 1` evaluates to `False`
## Implicit conversion can hurt sometimes
```py
@ -567,9 +607,44 @@ for i in x:
## Minor ones
- join() is a string operation instead of list operation. (sort of counterintuitive)
- `join()` is a string operation instead of list operation. (sort of counterintuitive)
- `[] = ()` is a semantically correct statement (unpacking an empty `tuple` into an empty `list`)
- No multicore support yet
## "Needle in a Haystack" bugs
This contains some of the potential bugs in you code that are very common but hard to detect.
### Initializing a tuple containing single element
```py
t = ('one', 'two')
for i in t:
print(i)
t = ('one')
for i in t:
print(i)
t = ()
print(t)
```
**Output:**
```py
one
two
o
n
e
tuple()
```
#### Explanation
* The correct statement for expected behavior is `t = ('one',)` or `t = 'one',` (missing comma) otherwise the interpreter considers `t` to be a `str` and iterates over it character by character.
* `()` is a special token and denotes empty `tuple`.
# Contributing
All patches are Welcome! Filing an issue first before submitting a patch will be appreciated :)