python

Pythonic

inspired from
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run python code online

zip/unzip

def unzip(tuples):
  if tuples:
    return [tuple(t[i] for t in tuples) for i, _ in enumerate(tuples[0])]
  else:
    return []

result:

>>> unzip( ((1, 2), (3, 4), (5, 6)) )
[(1, 3, 5), (2, 4, 6)]

in

long_string = "This is a very long string"
if "long" in long_string:
  print("Match found")

dict & counter

>>> from collections import Counter
>>> fruits = ['orange', 'banana', 'apple', 'orange', 'banana']
>>> Counter(fruits)
Counter({'orange': 2, 'banana': 2, 'apple': 1})

enumerate

x = ['a', 'b', 'c']

for index, item in enumerate(x):
  print(index, item)

array = [1, 2, 3, 4, 5]

for i, e in enumerate(array,0):
  print i, e
#0 1
#1 2
#2 3
#3 4
#4 5

NP:

for i in xrange(len(array)):
  print i, array[i]
#0 1
#1 2
#2 3
#3 4
#4 5

import local module

# A.py
def filter_items(items):
  for i in items:
    if i < 10:
      yield i


# B.py
from A import filter_items as A_filter_items

def filter_items(items):
  for i in items:
    if i <= 5:
      yield i

def do_something(items):
  x = A_filter_items(items)
  y = filter_items(items)
  return (x, y)

args & kwargs

def add(one, two):
  return one + two

my_list = [1, 2]
x = add(*my_list)  # x = 3

my_dict = {"one": 1, "two": 2}
y = add(**my_dict) #y = 3

itertools

>>> from itertools import zip_longest
>>> x = [1, 2, 3, 4]
>>> y = ['a', 'b', 'c']
>>> for i, j in zip_longest(x, y):
...     print(i, j)
...
1 a
2 b
3 c
4 None

one-line python code

>>> my_dict = {key: value for key, value in zip_longest(x,y)}
>>> my_dict
{1: 'a', 2: 'b', 3: 'c', 4: None}

slice

word = #some word
is_palindrome = word.find(word[-1::-1])

chain compare

P:
```
a = 3
b = 1
1 <= b <= a < 10  #True
```

NP:

a = 3
b = 1
b >= 1 and b <= a and a < 10 #True

boolean

name = 'Tim'
langs = ['AS3', 'Lua', 'C']
info = {'name': 'Tim', 'sex': 'Male', 'age':23 }

if name and langs and info:
  print('All True!')  #All True!

NP:

if name != '' and len(langs) > 0 and info != {}:
  print('All True!') #All True!

reverse

P:
```
def reverse_str( s ):
  return s[::-1]
```

NP:

def reverse_str( s ):
  t = ''
  for x in xrange(len(s)-1,-1,-1):
    t += s[x]
  return t

join in list

strList = ["Python", "is", "good"]
res =  ' '.join(strList) #Python is good

NP:

res = ''
for s in strList:
  res += s + ' '
#Python is good
#最后还有个多余空格

sum & max & min & time

numList = [1,2,3,4,5]
sum = sum(numList)    #sum = 15
maxNum = max(numList) #maxNum = 5
minNum = min(numList) #minNum = 1
from operator import mul
prod = reduce(mul, numList, 1) #prod = 120 默认值传1以防空列表报错

NP:

sum = 0
maxNum = -float('inf')
minNum = float('inf')
prod = 1
for num in numList:
  if num > maxNum:
    maxNum = num
  if num < minNum:
    minNum = num
  sum += num
  prod *= num
# sum = 15 maxNum = 5 minNum = 1 prod = 120

list comprehensions

l = [x*x for x in range(10) if x % 3 == 0]
# l = [0, 9, 36, 81]

NP:

l = []
for x in range(10):
  if x % 3 == 0:
    l.append(x*x)
# l = [0, 9, 36, 81]

default dict

dic = {'name':'Tim', 'age':23}

dic['workage'] = dic.get('workage',0) + 1
# dic = {'age': 23, 'workage': 1, 'name': 'Tim'}

NP:

if 'workage' in dic:
  dic['workage'] += 1
else:
  dic['workage'] = 1
# dic = {'age': 23, 'workage': 1, 'name': 'Tim'}

if...else...

for x in xrange(1,5):
  if x == 5:
    print 'find 5'
    break
else:
  print 'can not find 5!'
# can not find 5!

NP:

find = False
for x in xrange(1,5):
  if x == 5:
    find = True
    print 'find 5'
    break
if not find:
  print 'can not find 5!'
# can not find 5!

ternary operator

P:
```
a = 3

b = 2 if a > 2 else 1
# b = 2
```
NP:
```
if a > 2:
  b = 2
else:
  b = 1
# b = 2
```

dict & zip

keys = ['Name', 'Sex', 'Age']
values = ['Tim', 'Male', 23]

dic = dict(zip(keys, values))
# {'Age': 23, 'Name': 'Tim', 'Sex': 'Male'}

NP:

dic = {}
for i,e in enumerate(keys):
  dic[e] = values[i]
# {'Age': 23, 'Name': 'Tim', 'Sex': 'Male'}

hidden features

[!NOTE|label:references:]
Hidden features of Python [closed]

numbers

round

>>> str(round(1234.5678, -2))
'1200.0'
>>> str(round(1234.5678, 2))
'1234.57'

integer base

>>> int('10', 0)
10
>>> int('0x10', 0)
16
>>> int('010', 0)  # does not work on Python 3.x
8
>>> int('0o10', 0)  # Python >=2.6 and Python 3.x
8
>>> int('0b10', 0)  # Python >=2.6 and Python 3.x
2

in-place value swapping

>>> a = 10
>>> b = 5
>>> a, b
(10, 5)

>>> a, b = b, a
>>> a, b
(5, 10)

sum

from operator import add
print reduce(add, [1,2,3,4,5,6])

string

multi-line strings

>>> sql = "select * from some_table \
where id > 10"
>>> print sql
select * from some_table where id > 10

>>> sql = """select * from some_table
where id > 10"""
>>> print sql
select * from some_table where id > 10

>>> sql = ("select * from some_table " # <-- no comma, whitespace at end
           "where id > 10 "
           "order by name")
>>> print sql
select * from some_table where id > 10 order by name

>>> 'str' in 'string'
True
>>> 'no' in 'yes'
False
>>>

Join

''.join(list_of_strings)

set

>>> a = set([1,2,3,4])
>>> b = set([3,4,5,6])
>>> a | b # Union
{1, 2, 3, 4, 5, 6}
>>> a & b # Intersection
{3, 4}
>>> a < b # Subset
False
>>> a - b # Difference
{1, 2}
>>> a ^ b # Symmetric Difference
{1, 2, 5, 6}

slice operators

a = [1,2,3,4,5]
>>> a[::2]  # iterate over the whole list in 2-increments
[1,3,5]

or
```
>>> a[::-1]
[5,4,3,2,1]
```

>>> a = range(10)
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a[:5] = [42]
>>> a
[42, 5, 6, 7, 8, 9]
>>> a[:1] = range(5)
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> del a[::2]
>>> a
[1, 3, 5, 7, 9]
>>> a[::2] = a[::-2]
>>> a
[9, 3, 5, 7, 1]

reversed

for i in reversed([1, 2, 3]):
  print(i)

backslashes

>>> print repr(r"aaa\"bbb")
'aaa\\"bbb'

>>> print repr(r"C:\")
SyntaxError: EOL while scanning string literal
>>> print repr(r"C:\"")
'C:\\"'

args

Use _ instead of last printed item

>>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> _
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>>

*args & **kwargs

>>> g = lambda *args, **kwargs: args[0], kwargs['thing']
>>> g(1, 2, 3, thing='stuff')
(1, 'stuff')

def foo(a, b, c):
  print a, b, c

bar = (3, 14, 15)
foo(*bar)

function argument unpacking

def draw_point(x, y):
  # do some magic

point_foo = (3, 4)
point_bar = {'y': 3, 'x': 2}

draw_point(*point_foo)
draw_point(**point_bar)

conditional assignment

ternary operator

>>> 'ham' if True else 'spam'
'ham'
>>> 'ham' if False else 'spam'
'spam'

>>> True and 'ham' or 'spam'
'ham'
>>> False and 'ham' or 'spam'
'spam'

>>> [] if True else 'spam'
[]
>>> True and [] or 'spam'
'spam'

In [18]: a = True

In [19]: a and 3 or 4
Out[19]: 3

In [20]: a = False

In [21]: a and 3 or 4
Out[21]: 4

or
```
>>> (1 and [foo()] or [bar()])[0]
foo
0
```
or
```
>>> foo() if True or bar()
foo
0
```

conditional

x = 3 if (y == 1) else 2

x = 3 if (y == 1) else 2 if (y == -1) else 1

(func1 if y == 1 else func2)(arg1, arg2)

x = (class1 if y == 1 else class2)(arg1, arg2)

[(x, y) for x in range(4) if x % 2 == 1 for y in range(4)]
[(1, 0), (1, 1), (1, 2), (1, 3), (3, 0), (3, 1), (3, 2), (3, 3)]

x = 3 if (y == 1) else 2                is equvalent to                 x = y == 1 and 3 or 2
x = 0 if True else 1                    is equvalent to                 x = True and 0 or 1

foo = [x for x in xrange(10) if x % 2 == 0]

equal to

foo = []
for x in xrange(10):
  if x % 2 == 0:
    foo.append(x)

dict comprehensions, manual

>>> {i: i**2 for i in range(5)}
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

set comprehensions

wiki manual

>>> {i**2 for i in range(5)}
set([0, 1, 4, 16, 9])

list & dics

zip

a = [(1,2), (3,4), (5,6)]
zip(*a)
# [(1, 3, 5), (2, 4, 6)]

>>> dict([ ('foo','bar'),('a',1),('b',2) ])
{'a': 1, 'b': 2, 'foo': 'bar'}

>>> names = ['Bob', 'Marie', 'Alice']
>>> ages = [23, 27, 36]
>>> dict(zip(names, ages))
{'Alice': 36, 'Bob': 23, 'Marie': 27}

>>> t1 = (0,1,2,3)
>>> t2 = (7,6,5,4)
>>> [t1,t2] == zip(*zip(t1,t2))
True

In [15]: t1 = (1, 2, 3)
In [16]: t2 = (4, 5, 6)
In [17]: dict (zip(t1,t2))
Out[17]: {1: 4, 2: 5, 3: 6}

>>> l=[(1,2),(3,4)]
>>> [a+b for a,b in l ]
[3,7]

list & sum

>>> l = [[1, 2, 3], [4, 5], [6], [7, 8, 9]]
>>> sum(l, [])
[1, 2, 3, 4, 5, 6, 7, 8, 9]

nested list

[(i,j) for i in range(3) for j in range(i) ]

((i,j) for i in range(4) for j in range(i) )

enumerate

>>> a = ['a', 'b', 'c', 'd', 'e']
>>> for index, item in enumerate(a): print index, item
...
0 a
1 b
2 c
3 d
4 e

>>> l = ["spam", "ham", "eggs"]
>>> list(enumerate(l))
>>> [(0, "spam"), (1, "ham"), (2, "eggs")]
>>> list(enumerate(l, 1))
>>> [(1, "spam"), (2, "ham"), (3, "eggs")]

generate list

>>> from functools import partial
>>> bound_func = partial(range, 0, 10)
>>> bound_func()
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> bound_func(2)
[0, 2, 4, 6, 8]

dict's constructor

>>> dict(foo=1, bar=2)
{'foo': 1, 'bar': 2}

>>> a = {}
>>> b = a.setdefault('foo', 'bar')
>>> a
{'foo': 'bar'}
>>> b
'bar

dict's get

t = {1: 'a'}
>>> test[2]

Traceback (most recent call last):
  File "<pyshell#158>", line 1, in <module>
    test[2]
KeyError: 2
>>> test.get(2)
>>> test.get(1)
'a'
>>> test.get(2) == None
True
>>> test.get(2, 'some') == 'some'
True

copy list

>>> x = [1,2,3]
>>> y = x[:]
>>> y.pop()
3
>>> y
[1, 2]
>>> x
[1, 2, 3]

replace list

>>> x = [1,2,3]
>>> y = x
>>> y[:] = [4,5,6]
>>> x
[4, 5, 6]

generators objects

x = [n for n in foo if bar(n)]

>>> n = ((a,b) for a in range(0,2) for b in range(4,6))
>>> for i in n:
...   print i

(0, 4)
(0, 5)
(1, 4)
(1, 5)

generator & iteration

iteration & constructor (yield)

>>> def g(n):
...     for i in range(n):
...             yield i **2
>>> t = g(5)
>>> t.next()
0
>>> t.next()
1
>>> t.next()
4
>>> t.next()
9
>>> t.next()
16
>>> t.next()
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
StopIteration

def fab(max):
  a,b = 0,1
  while a < max:
    yield a
    a, b = b, a+b

>>> for i in fab(20):
...     print i,",",
...
0 , 1 , 1 , 2 , 3 , 5 , 8 , 13 ,

>>> i = (1,2,3,4,5,6,7,8,9,10) # or any iterable object
>>> iterators = [iter(i)] * 2
>>> iterators[0].next()
1
>>> iterators[1].next()
2
>>> iterators[0].next()
3

def grouper(n, iterable, fillvalue=None):
  "grouper(3, 'ABCDEFG', 'x') --> ABC DEF Gxx"
  args = [iter(iterable)] * n
  return izip_longest(fillvalue=fillvalue, *args)

>>> from itertools import *
>>> l = [[1, 2], [3, 4]]
>>> list(chain(*l))
[1, 2, 3, 4]

def create_printers(n):
  for i in xrange(n):
    def printer(i=i): # Doesn't work without the i=i
      print i
    yield printer

statement

for...else...

for i in foo:
  if i == 0:
    break
else:
  print("i was never 0")

found = False
for i in foo:
  if i == 0:
    found = True
    break
if not found:
  print("i was never 0")

context managers and the "with" statement

from __future__ import with_statement

with open('foo.txt', 'w') as f:
  f.write('hello!')

try...except...elese...finally

try:
  put_4000000000_volts_through_it(parrot)
except Voom:
  print "'E's pining!"
else:
  print "This parrot is no more!"
finally:
  end_sketch()

funcs

dir

>>> dir("foo")
['__add__', '__class__', '__contains__', (snipped a bunch), 'title',
 'translate', 'upper', 'zfill']

help

>>> help("foo".upper)
  Help on built-in function upper:

upper(...)
  S.upper() -> string

  Return a copy of the string S converted to uppercase.

convenient web-browser controller

>>> import webbrowser
>>> webbrowser.open_new_tab('http://www.stackoverflow.com')

built-in http server

python -m SimpleHTTPServer 8000

an interpreter within the interpreter

$ python
Python 2.5.1 (r251:54863, Jan 17 2008, 19:35:17)
[GCC 4.0.1 (Apple Inc. build 5465)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> shared_var = "Set in main console"
>>> import code
>>> ic = code.InteractiveConsole({ 'shared_var': shared_var })
>>> try:
...     ic.interact("My custom console banner!")
... except SystemExit, e:
...     print "Got SystemExit!"
...
My custom console banner!
>>> shared_var
'Set in main console'
>>> shared_var = "Set in sub-console"
>>> import sys
>>> sys.exit()
Got SystemExit!
>>> shared_var
'Set in main console'

pretty print

>>> import pprint
>>> stuff = sys.path[:]
>>> stuff.insert(0, stuff)
>>> pprint.pprint(stuff)
[<Recursion on list with id=869440>,
 '',
 '/usr/local/lib/python1.5',
 '/usr/local/lib/python1.5/test',
 '/usr/local/lib/python1.5/sunos5',
 '/usr/local/lib/python1.5/sharedmodules',
 '/usr/local/lib/python1.5/tkinter']

from __future__ import print_function

mylist = ['foo', 'bar', 'some other value', 1,2,3,4]
print(*mylist)

class & module

bash

python -c"import os; print(os.getcwd());"

assertion

>>> try:
...     assert []
... except AssertionError:
...     print "This list should not be empty"
This list should not be empty

import

try:
  import json
except ImportError:
  import simplejson as json

create new types

>>> NewType = type("NewType", (object,), {"x": "hello"})
>>> n = NewType()
>>> n.x
"hello"

>>> class NewType(object):
>>>     x = "hello"
>>> n = NewType()
>>> n.x
"hello"

Manipulating sys.modules

>>> import sys
>>> import ham
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
ImportError: No module named ham

# Make the 'ham' module available -- as a non-module object even!
>>> sys.modules['ham'] = 'ham, eggs, saussages and spam.'
>>> import ham
>>> ham
'ham, eggs, saussages and spam.'

# Now remove it again.
>>> sys.modules['ham'] = None
>>> import ham
Traceback (most recent call last):

>>> import os
# Stop future imports of 'os'.
>>> sys.modules['os'] = None
>>> import os
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
ImportError: No module named os
# Our old imported module is still available.
>>> os
<module 'os' from '/usr/lib/python2.5/os.pyc'>

Others

not hidden but still nice

import os.path as op

root_dir = op.abspath(op.join(op.dirname(__file__), ".."))

be careful with mutable default arguments

>>> def foo(x=[]):
...     x.append(1)
...     print x
...
>>> foo()
[1]
>>> foo()
[1, 1]
>>> foo()
[1, 1, 1]

>>> def foo(x=None):
...     if x is None:
...         x = []
...     x.append(1)
...     print x
>>> foo()
[1]
>>> foo()
[1]

PEP8 -- Style Guide for Python Code

Indentation

yes:

# Aligned with opening delimiter.
foo = long_function_name(var_one, var_two,
                         var_three, var_four)

# More indentation included to distinguish this from the rest.
def long_function_name(
    var_one, var_two, var_three,
    var_four):
  print(var_one)

# Hanging indents should add a level.
foo = long_function_name(
    var_one, var_two,
    var_three, var_four)

no:

# Arguments on first line forbidden when not using vertical alignment.
foo = long_function_name(var_one, var_two,
    var_three, var_four)

# Further indentation required as indentation is not distinguishable.
def long_function_name(
    var_one, var_two, var_three,
    var_four):
  print(var_one)

optional

# Hanging indents *may* be indented to other than 4 spaces.
foo = long_function_name(
  var_one, var_two,
  var_three, var_four)

if statemant

# No extra indentation.
if (this_is_one_thing and
    that_is_another_thing):
  do_something()

# Add a comment, which will provide some distinction in editors
# supporting syntax highlighting.
if (this_is_one_thing and
    that_is_another_thing):
    # Since both conditions are true, we can frobnicate.
  do_something()

# Add some extra indentation on the conditional continuation line.
if (this_is_one_thing
    and that_is_another_thing):
  do_something()o

list

my_list = [
        1, 2, 3,
        4, 5, 6,
        ]
result = some_function_that_takes_arguments(
        'a', 'b', 'c',
        'd', 'e', 'f',
        )

my_list = [
        1, 2, 3,
        4, 5, 6,
]
result = some_function_that_takes_arguments(
        'a', 'b', 'c',
        'd', 'e', 'f',
)

Maximum Line Length

yes:

with open('/path/to/some/file/you/want/to/read') as file_1, \
         open('/path/to/some/file/being/written', 'w') as file_2:
  file_2.write(file_1.read())

Should a Line break before or after a binary operator?

no: operators sit far away from their operands

income = (gross_wages +
          taxable_interest +
          (dividends - qualified_dividends) -
          ira_deduction -
          student_loan_interest)

yes: easy to match operators with operands

income = (gross_wages
          + taxable_interest
          + (dividends - qualified_dividends)
          - ira_deduction
          - student_loan_interest)

imports

no:
```
import sys, os
```
yes:
```
import os
import sys
```
bad:
```
import <module> from *
```

absolute imports are recommended

import mypkg.sibling
from mypkg import silbing
from mypkg.sibling import example

explicit relative imports are acceptable

from . import sibling
from .sibling import example

import a class from a class-containing module

from myclass import MyClass
from foo.bar.yourclass import YourClass

local name classes

import myclass
import foo.bar.yourclass

# use "myclass.MyClass" or "foo.bar.yourclass.YourClass"

module Level dunder names

Module level "dunder" names with two leading and two trailing underscores, such as __all__, __author__, __version__, etc

yes:

"""This is the example module.

This module does stuff.
"""

from __future__ import barry_as_FLUFL

__all__ = ['a', 'b', 'c']
__version__ = '0.1'
__author__ = 'Cardinal Biggles'

import os
import sys

whitespace in expressions and statements

no:
```
spam( ham[ 1 ], { eggs: 2 } )
```
yes:
```
spam(ham[1], {eggs: 2})
```

no:
```
if x == 4 : print x , y ; x , y = y , x
```
yes:
```
if x == 4; print x, y; x, y = y, x
```

no:

ham[lower + offset:upper + offset]
ham[1: 9], ham[1 :9], ham[1:9 :3]
ham[lower : : upper]
ham[ : upper ]

yes:

ham[1:9], ham[1:9:3], ham[:9:3], ham[1::3], ham[1:9:]
ham[lower:upper], ham[lowser:pper:], ham[lower::step]
ham[lower+offset : upper+offset]
ham[: upper_fn(x) : setp_fn(x)], ham[:: setp_fn(x)]
ham[lower + offset : upper + offset]

no:
```
spam (1)
```
yes:
```
spam(1)
```

no:
```
dct ['key'] = lst [index]
```
yes:
```
dct['key'] = lst[index]
```

no:

x             = 1
y             = 2
long_variable = 3

yes:
```
x = 1
y = 2
long_variable = 3
```

other recommendations

no:

i=i+1
submitted +=1
x = x * 2 - 1
hypot2 = x * x + y * y
c = (a + b) * (a - b)

yes:

i = i + 1
submitted += 1
x = x*2 - 1
hypot2 = x*x + y*y
c = (a+b) * (a-b)

no:

def complex(real, imag = 0.0):
  return magic(r = real, i = imag)

yes:

def complex(real, imag=0.0):
  return magic(r=real, i=imag)

no:

def munge(input:AnyStr): ...
def munge()->PosInt: ...

yes:

def munge(input: AnyStr): ...
def munge() -> AnyStr: ...

no:

def munge(input: AnyStr=None): ...
def munge(input: AnyStr, limit = 1000): ...

yes:

def munge(sep: AnyStr = None): ...
def munge(input: AnyStr, sep: AnyStr = None, limit=1000): ...

rather NO

if foo == 'blah': do_blah_thing()
do_one(); do_two(); do_three()

yes:

if foo == 'blah':
  do_blah_thing()
do_one()
do_two()
do_three()

DEFINITELY NO

if foo == 'blah': do_blah_thing()
else: do_non_blah_thing()

try: something()
finally: cleanup()

do_one(); do_two(); do_three(long, argument,
                             list, like, this)

if foo == 'blah': one(); two(); three()

yes:

if foo == 'blah': do_blah_thing()
for x in lst: total += x
while t < 10: t = delay()

documentation strings

yes:

"""Return a foobang

Optional plotz says to frobnicate the bizbaz first.
"""

Programming Recommendations

no:
```
if not foo is None:
```
yes:
```
if foo is not None:
```

no:
```
f = lambda x: 2*x
```
yes:
```
def f(x): return 2*x
```

no:

try:
  # Too broad!
  return handle_value(collection[key])
expect KeyError:
  # Will also catch KeyError raised by handle_value()
  return key_not_found(key)

yes:

try:
  value = collection[key]
except KeyError:
  return key_not_found(key)
else:
  return handle_value(value)

no:

with conn:
  do_stuff_in_transaction(conn)

yes:

with conn.begin_transaction():
  do_stuff_in_transaction(conn)

no:

def foo(x):
  fi x >= 0:
    return math.sqrt(x)

def bar(x):
  if x < 0:
    return
  return math.sqrt(x)

yes:

def foo(x):
  if x >= 0:
    return math.sqrt(x)
  else:
    return None

def bar(x):
  if x < 0:
    return None
  return math.sqrt(x)

no:
```
if foo[:3] == 'bar':
```
yes:
```
if foo.startwith('bar'):
```

no:
```
if type(obj) is type(1):
```
yes:
```
if isinstance(obj, int):
```

no:
```
if len(seq):
if not len(seq):
```
yes:
```
if not seq:
if seq:
```

no:
```
if greeting == True:
```
yes:
```
if greeting:
```
worse:
```
if greeting is True:
```

PEP8 Error/Warning Code

E1 Indentation

E101 indentation contains mixed spaces and tabs
E111 indentation is not a multiple of four
E112 expected an indented block
E113 unexpected indentation
E114 indentation is not a multiple of four (comment)
E115 expected an indented block (comment)
E116 unexpected indentation (comment)
E121 (*^) continuation line under-indented for hanging indent
E122 (^) continuation line missing indentation or outdented
E123 (*) closing bracket does not match indentation of opening bracket’s line
E124 (^) closing bracket does not match visual indentation
E125 (^) continuation line with same indent as next logical line
E126 (*^) continuation line over-indented for hanging indent
E127 (^) continuation line over-indented for visual indent
E128 (^) continuation line under-indented for visual indent
E129 (^) visually indented line with same indent as next logical line
E131 (^) continuation line unaligned for hanging indent
E133 (*) closing bracket is missing indentation

E2 Whitespace

E201 whitespace after ‘(‘
E202 whitespace before ‘)’
E203 whitespace before ‘:’
E211 whitespace before ‘(‘
E221 multiple spaces before operator
E222 multiple spaces after operator
E223 tab before operator
E224 tab after operator
E225 missing whitespace around operator
E226 (*) missing whitespace around arithmetic operator
E227 missing whitespace around bitwise or shift operator
E228 missing whitespace around modulo operator
E231 missing whitespace after ‘,’, ‘;’, or ‘:’
E241 (*) multiple spaces after ‘,’
E242 (*) tab after ‘,’
E251 unexpected spaces around keyword / parameter equals
E261 at least two spaces before inline comment
E262 inline comment should start with ‘# ‘
E265 block comment should start with ‘# ‘
E266 too many leading ‘#’ for block comment
E271 multiple spaces after keyword
E272 multiple spaces before keyword
E273 tab after keyword
E274 tab before keyword

E3 Blank line

E301 expected 1 blank line, found 0
E302 expected 2 blank lines, found 0
E303 too many blank lines (3)
E304 blank lines found after function decorator

E4 Import

E401 multiple imports on one line
E402 module level import not at top of file

E5 Line length

E501 (^) line too long (82 > 79 characters)
E502 the backslash is redundant between brackets

E7 Statement

E701 multiple statements on one line (colon)
E702 multiple statements on one line (semicolon)
E703 statement ends with a semicolon
E704 (*) multiple statements on one line (def)
E711 (^) comparison to None should be ‘if cond is None:’
E712 (^) comparison to True should be ‘if cond is True:’ or ‘if cond:’
E713 test for membership should be ‘not in’
E714 test for object identity should be ‘is not’
E721 (^) do not compare types, use ‘isinstance()’
E731 do not assign a lambda expression, use a def

E9 Runtime

E901 SyntaxError or IndentationError
E902 IOError

W1 Indentation warning

W191 indentation contains tabs

W2 Whitespace warning

W291 trailing whitespace
W292 no newline at end of file
W293 blank line contains whitespace

W3 Blank line warning

W391 blank line at end of file

W5 Line break warning

W503 line break occurred before a binary operator

W6 Deprecation warning

W601 .has_key() is deprecated, use ‘in’
W602 deprecated form of raising exception
W603 ‘<>’ is deprecated, use ‘!=’
W604 backticks are deprecated, use ‘repr()’

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