Python中的陷阱与技巧 Python中的一些陷阱与技巧小结

drinks = ["coffee", "tea", "milk", "water"]
for drink in drinks:
  print("thirsty for", drink)
#thirsty for coffee
#thirsty for tea
#thirsty for milk
#thirsty for water

drinks = ["coffee", "tea", "milk", "water"]
for index, drink in enumerate(drinks):
  print("Item {} is {}".format(index, drink))
#Item 0 is coffee
#Item 1 is tea
#Item 2 is milk
#Item 3 is water

# deduplicate a list *fast*
print(set(["ham", "eggs", "bacon", "ham"]))
# {'bacon', 'eggs', 'ham'}
 
# compare lists to find differences/similarities
# {} without "key":"value" pairs makes a set
menu = {"pancakes", "ham", "eggs", "bacon"}
new_menu = {"coffee", "ham", "eggs", "bacon", "bagels"}
 
new_items = new_menu.difference(menu)
print("Try our new", ", ".join(new_items))
# Try our new bagels, coffee
 
discontinued_items = menu.difference(new_menu)
print("Sorry, we no longer have", ", ".join(discontinued_items))
# Sorry, we no longer have pancakes
 
old_items = new_menu.intersection(menu)
print("Or get the same old", ", ".join(old_items))
# Or get the same old eggs, bacon, ham
 
full_menu = new_menu.union(menu)
print("At one time or another, we've served:", ", ".join(full_menu))
# At one time or another, we've served: coffee, ham, pancakes, bagels, bacon, eggs

LightObject = namedtuple('LightObject', ['shortname', 'otherprop'])
m = LightObject()
m.shortname = 'athing'
> Traceback (most recent call last):
> AttributeError: can't set attribute

LightObject = namedtuple('LightObject', ['shortname', 'otherprop'])
n = LightObject(shortname='something', otherprop='something else')
n.shortname 
# something
collections.defaultdict

login_times = {}
for t in logins:
  if login_times.get(t.username, None):
    login_times[t.username].append(t.datetime)
  else:
    login_times[t.username] = [t.datetime]

login_times = collections.defaultdict(list)
for t in logins:
  login_times[t.username].append(t.datetime)

from datetime import datetime
class Event(object):
  def __init__(self, t=None):
  if t is None:
    self.time = datetime.now()
  else:
    self.time = t
 
events = collections.defaultdict(Event)
 
for e in user_events:
  print(events[e.name].time)

normal_dict = {
  'a': {
    'b': {
      'c': {
        'd': {
          'e': 'really really nested dict'
        }
      }
    }
  }
}
 
from addict import Dict
addicted = Dict()
addicted.a.b.c.d.e = 'really really nested'
print(addicted)
# {'a': {'b': {'c': {'d': {'e': 'really really nested'}}}}}

from collections import defaultdict
default = defaultdict(dict)
default['a']['b']['c']['d']['e'] = 'really really nested dict' 
# fails

try:
  
# get API data
  data = db.find(id='foo') 
# may raise exception
  
# manipulate the data
  db.add(data)
  
# save it again
  db.commit() 
# may raise exception
except Exception:
  
# log the failure
  db.rollback()
 
db.close()

try:
  
# get API data
  data = db.find(id='foo') 
# may raise exception
except Exception:
  
# log the failure and bail out
  log.warn("Could not retrieve FOO")
  return
 
# manipulate the data
db.add(data)

try:
  db.commit() 
# may raise exception
except Exception:
  log.warn("Failure committing transaction, rolling back")
  db.rollback()
else:
  log.info("Saved the new FOO")
finally:
  db.close()

try:
  
# attempt to acquire a resource
  db.commit()
except Exception:
  
# If it fails, clean up anything left behind
  log.warn("Failure committing transaction, rolling back")
  db.rollback()
else:
  
# If it works, perform actions
  
# In this case, we just log success
  log.info("Saved the new FOO")
finally:
  
# Clean up
  db.close()
# Program complete

db = db_library.connect("fakesql://")
# as a function
commit_or_rollback(db)
 
# context manager
with transaction("fakesql://") as db:
  
# retrieve data here
  
# modify data here

class DatabaseTransaction(object):
  def __init__(self, connection_info):
    self.conn = db_library.connect(connection_info)
 
  def __enter__(self):
    return self.conn

def __exit__(self, exc_type, exc_val, exc_tb):
    if exc_type is not None:
      self.conn.rollback()
 
    try:
      self.conn.commit()
    except Exception:
      self.conn.rollback()
    finally:
      self.conn.close()

# context manager
with DatabaseTransaction("fakesql://") as db:
  
# retrieve data here
  
# modify data here

def my_generator(v):
  yield 'first ' + v
  yield 'second ' + v
  yield 'third ' + v
 
print(my_generator('thing'))
# <generator object my_generator at 0x....>

for value in my_generator('thing'):
  print value
 
# first thing
# second thing
# third thing
 
gen = my_generator('thing')
next(gen)
# 'first thing'
next(gen)
# 'second thing'
next(gen)
# 'third thing'
next(gen)
# raises StopIteration exception

def fib_generator():
  a = 0
  b = 1
  while True:
    yield a
    a, b = b, a + b

min = 10000
for number in fib_generator():
  if number > min:
    print(number, "is the first fibonacci number over", min)
    break

GET http://scream-about-food.com/search?q=coffee
{
  "results": [
    {"name": "Coffee Spot",
     "screams": 99
    },
    {"name": "Corner Coffee",
     "screams": 403
    },
    {"name": "Coffee Moose",
     "screams": 31
    },
    {...}
  ]
  "more": true,
  "_next": "http://scream-about-food.com/search?q=coffee?p=2"
}

import requests
 
api_url = "http://scream-about-food.com/search?q={term}"
 
def infinite_search(term):
  url = api_url.format(term)
  while True:
    data = requests.get(url).json()
 
    for place in data['results']:
      yield place
 
    
# end if we've gone through all the results
    if not data['more']: break
 
    url = data['_next']

# pass a number to start at as the second argument if you don't want
# zero-indexing
for number, result in enumerate(infinite_search("coffee"), 1):
  if result['name'] == "The Coffee Stain":
    print("Our restaurant, The Coffee Stain is number ", number)
    return
print("Our restaurant, The Coffee Stain didnt't show up at all! :(")

for result in infinite_search("coffee"):
  if result['name'] == "The Coffee Stain":
    print("Our restaurant, The Coffee Stain is number ", result['number'])
    return
print("Our restaurant, The Coffee Stain didn't show up at all! :(")
Divided Over Division

print "hello" 
# Python 2
print("hello") 
# Python 3
 
from __future__ import print_function
print("hello") 
# Python 2
print("hello") 
# Python 3

print(1 / 3) 
# Python 2
# 0
print(1 / 3) 
# Python 3
# 0.3333333333333333
print(1 // 3) 
# Python 3
# 0