python.md

Python CHEATSHEET

Date and Time

• subtract time from a datetime object:

import datetime
t = datetime.datetime(2018,10,10,12,12,0)
new_t = t.replace(hour=t.hour+1)

• format a datetime object (click HERE for the format string reference):

import datetime
t = datetime.datetime(2018,10,10,12,12,0)
t.strftime('%Y-%m-%d')

• get a datetime obj from a string:

datetime_object = datetime.strptime('Jun 1 2005  1:33PM', '%b %d %Y %I:%M%p')

loop through files in filesystem

import os

for subdir, dirs, files in os.walk(rootdir):
    for file in files:
        #print os.path.join(subdir, file)
        filepath = subdir + os.sep + file

loop through csv file

with open('eggs.csv', 'rb') as csvfile:
     spamreader = csv.reader(csvfile, delimiter=' ', quotechar='|')
     for row in spamreader:
         print(', '.join(row))

double for loop / Double Iteration in List Comprehension

from: https://stackoverflow.com/questions/1198777/double-iteration-in-list-comprehension

# double for loop
list_of_words = []
for sentence in text:
    for word in sentence:
       list_of_words.append(word)

# list comprehension with two iterations
[word for sentence in text for word in sentence]

[target_item for big_loop_iterable_item in big_loop_collection for target_item in big_loop_iterable_item]

# double for loop
list_of_people = []
for org in org_list:
    for person in org.person_list:
       list_of_people.append(person)

# list comprehension with two iterations
[person for org.person_list in org_list for person in person_list]

Check python bit version

#For 32 bit it will return 32 and for 64 bit it will return 64
import struct
print(struct.calcsize("P") * 8)

pip Commands

#list installed packages
pip freeze

#check pip version
pip --version

#upgrade pip
pip install -U pip

#verify installed packages have compatible dependencies
pip check

#You can also put a list of packages (one per line) in a text file and install all of them at once using the 
#-r or --requirement flag.
pip install -r requirements.txt

#Other commands 
https://pip.pypa.io/en/stable/reference/

Pandas

• repeat something: np.repeat(x, n)

dataframes

• get the first column of a DF: df.iloc[:,0]

• iterate a DF over rows:

for index, row in df.iterows():
    print(row)

Series

Matplotlib

• horizontal line: plt.axhline(0, color='white')
• vertical line: plt.axvline(0, color='white')

SciPy

• Find a P-Value from a t-Score:

  # find p-value
  import scipy.stats
  # left-tailed
  scipy.stats.t.sf(abs(1.87), df=24)
  # right-tailed
  scipy.stats.t.sf(abs(-.77), df=15)
  # two-tailed
  scipy.stats.t.sf(abs(1.87), df=24) * 2
  

• Find t* critical value from significance level (q) and df:

  # find T critical value
  q = 0.01
  df = 179
  import scipy.stats
  # Right-tailed
  scipy.stats.t.ppf(q=1-q,df=df)
  # left-tailed
  scipy.stats.t.ppf(q=q,df=df)
  # two-tailed
  scipy.stats.t.ppf(q=1-q/2,df=df)
  

• Find a P-Value from a chisq-Score:

  # find p-value
  import scipy.stats
  # left-tailed
  scipy.stats.t.sf(abs(1.87), df=24)
  # right-tailed
  scipy.stats.t.sf(abs(-.77), df=15)
  # two-tailed
  scipy.stats.t.sf(abs(1.87), df=24) * 2