A Newb Learns Python

In [11]: issubclass(bool, int) Out[11]: True In [12]: isinstance(True, int) Out[12]: True In [13]: isinstance(False, int) Out[13]: True

Pythons conditional syntax (e.g. "if not even" or "if foo in bar") is usually pretty natural. However, it is so natural that you may find yourself wanting to use it to perform set operations, bad idea...

It took me far too long to realize that the Python interpreter supports 'reverse-i-search' just like in Bash. Just the standard "Ctrl+R" shortcut like in Bash.

For those not familiar with it, reverse-i-search lets you interactively search your command history. Instead of hitting up a bunch of times find a command you used a while ago, just hit Ctrl+R and start typing text in the command. It will match the last command you typed that contains that text. If you want to go to the previous match just hit Ctrl+R again.

>>> a = 1 >>> b = 2 >>> c = 3 >>> d = a + b + c >>> >>> e = 4 >>> f = 5 (reverse-i-search)`d': d = a + b + c

If you need readable unix timestamps stop mucking with strftime and its impossible to remember formatting, just use time.ctime()

I recently worked on writing a script that included an expensive query on the database. Every time I ran the script it did the query over again. Rather than hammer the database I decided I would cache the result in a file until I had finished writing and debugging the script. Below is the simplified version of how it worked.

import os import cPickle as pickle def cached_data(): cache_path = "/tmp/cached-data.pickle" if not os.path.exists(cache_path): # The cache doesn't exist, create it and populate it result = generate_data() cache_file = open(cache_path,'wb') # Write it to the result to the file as a pickled object # Use the binary protocol for better performance pickle.dump(result, cache_file, protocol=1) cache_file.close() return pickle.load(open(cache_path,'rb')) def generate_data(): """ This function actually generates the data when it isn't cached. The data generated is often expensive to compute so caching helps with performance. """ return []

>>> nested = ["I", ["have", "some", "nested"], ["entries"], ["eek"]] >>> import itertools >>> list(itertools.chain(*nested)) ['I', 'have', 'some', 'nested', 'entries', 'eek']

>>> keys = ["I", "am", "awesome"] >>> d = {k : True for k in keys} >>> d {'I': True, 'awesome': True, 'am': True} >>> >>> keys = [1, 2, 3, 4] >>> even = { k : k**k for k in keys if k % 2 == 0} >>> even {2: 4, 4: 256}

>>> values = [3,2,1,3,2] >>> list(set(values)) [1, 2, 3]

I keep a Python file around with a bunch of imports, some helpful utility methods and a bunch of code to setup database connections. Rather than having to copy-paste that code into my interpreter each time I start a new one up,  you can just use "execfile" to bring it in all at once.

For example say my utils file looks like: 

import os, sys from time import time from datetime import datetime def timeago(t): return str(datetime.now() - datetime.fromtimestamp(t))

I can just pop open a python interpreter and type:

$ python >>> execfile("/home/jonathan/bin/pyutils.py") >>> time() 1321479210.8605671 >>> timeago(time() - 900000) '10 days, 10:00:00.000008' >>>

Why use a website when Python can pretty print json right in your terminal...

>>> cat search.json {"responseHeader":{"status":0,"QTime":0},"response":{"numFound":86295,"start":0,"docs":[{"artist":"Explosions in the Sky","id":"6-1010373","image":"67660114.jpg","reach":768165,"resid":1010373,"restype":6,"weight":1.0},{"artist":"Skid Row","id":"6-1000620","image":"796810.jpg","reach":515805,"resid":1000620,"restype":6,"weight":1.0}]}} >>> cat search.json | python -m json.tool { "response": { "docs": [ { "artist": "Explosions in the Sky", "id": "6-1010373", "image": "67660114.jpg", "reach": 768165, "resid": 1010373, "restype": 6, "weight": 1.0 }, { "artist": "Skid Row", "id": "6-1000620", "image": "796810.jpg", "reach": 515805, "resid": 1000620, "restype": 6, "weight": 1.0 } ], "numFound": 86295, "start": 0 }, "responseHeader": { "QTime": 0, "status": 0 } }

Is the JSON already in your clipboard? No need to put it in a file, just use the builtin "pbpaste" command-line tool.

I wondered if there was a helper method for this...

Ever forget the name of that variable/class you defined in the interpreter? Just use locals().

It's important to note that == checks equality while "is" checks that they are the same object.

Just print the doc string...

Need to maintain a list of the last n objects you've seen? Stop writing that while loop over and over and use a deque!