Mind-blowing Python tips
0 - Loop over a range of numbers
Use range instead of xrange.
In python3, the former creates an iterator which produces the values one at
a time making it much more efficient and fast.
nums = [0,2, 34, 55, 32]
for i in range(nums):
print i
1 - Looping backwards
.reversed use Just
names = ["Case", "Molly", "Armitage", "Maelcum"]
for name in reversed(names):
print name
2 - Looping over a list and its indices
To keep track of the index of each item in a collection, enumerate is your buddy.
names = ["Case", "Molly", "Armitage", "Maelcum"]
for index, name in enumerate(names):
print index, name
3 - Looping over two lists simultaneously
Yeah you could use zip, but izip is faster, so use that instead.
from itertools import izip
names = ["Case", "Molly", "Armitage", "Maelcum"]
ages = [23, 27, 41, 24]
for name, age in izip(names, ages):
print name, age
4 - Looping over a sorted list
You can sort out the list first then loop through it, or you could use sorted.
names = ["Case", "Molly", "Armitage", "Maelcum"]
for name in sorted(names):
print name
And BAM, you’re … sorted.
5 - Call a function until a sentinel value is returned
To do that, use iter().
Bad example:
Loop over a file containing a list of names until the loop returns an empty string, in which case we break out of it.
names = []
while True:
name = file.read(32)
if name = "":
break
names.append(name)
Beautiful example:
In this case, we call a function (f.read) until it returns the sentinel value passed as a second argument to iter. That way we avoid having to make the unnecessary if check.
for name in iter( partial(f.read(32)), ""):
print name
6 - Looping over a dictionary
The normal way to do it:
molly = { "name": "Molly Millions", "Age": 27, "Occupation": "Professional Killer"}
for key in molly:
print key
If you wish to mutate the data, prefer dict.keys().
molly = { "name": "Molly Millions", "Age": 27, "Occupation": "Professional Killer"}
for key in molly.keys():
# do the mutation
7 - Looping over a dict keys AND values
Don’t do this:
molly = { "name": "Molly Millions", "Age": 27, "Occupation": "Professional Killer"}
for key in molly:
print molly[key]
It’s slow because we have to rehash the dictionary and do a lookup everytime.
Instead choose iteritems():
molly = { "name": "Molly Millions", "Age": 27, "Occupation": "Professional Killer"}
for key, value in molly.iteritems():
print key, value
8 - Create a dict out of two lists
Just instantiate a new dict with two zipped lists. Real magic.
from itertools import izip
names = ["Case", "Molly", "Armitage", "Maelcum"]
ages = [23, 27, 41, 24]
characters = dict(izip(names, ages))
9 - Use named tuples for returning multiple values
Like in the case of an API response in Flask.
from collections import namedtuple
Response = namedtuple('APIResponse', ['status_code', 'body', 'headers'])
@app.route('/users/1'):
try:
user = db.getuserbyid(1)
except:
return Response(404, user.notfound(), {'content-type': 'application/json'}
else:
return Response(200, user.json(), {'content-type': 'application/json'}
Other
- Always clarify function calls by using keyword arguments
If you learned something from this article, share it with your co-workers and fellow hackers. If you notice any typo, error etc let me know on twitter.