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Python tricks

Default header

import numpy as np
import scipy as sp
import matplotlib.pylab as pl


import time

# parallel computations
import multiprocessing as mp

Parallel computations

Example:

import multiprocessing as mp

def func(c):
    return c**2
    
nproc=4
pool = mp.Pool(nproc)

l=[i for i in range(100)]

# parallel map
res=pool.map(func,l)

Note that the function func has to be declared in the root of the module (pickle problem if not)

Cython compile

$ cython -a yourmod.pyx

Final compilation:

$ gcc -shared -pthread -fPIC -fwrapv -O2 -Wall -fno-strict-aliasing \
      -I/usr/include/python2.7 -o yourmod.so yourmod.c

or create a setup.py of the form

from distutils.core import setup
from Cython.Build import cythonize

setup(
    name = "My hello app",
    ext_modules = cythonize('hello.pyx'),  # accepts a glob pattern
)

then compile using the command:

$ python setup.py build_ext --inplace

Argument parser

Example

import argparse

# initialise the parser
parser = argparse.ArgumentParser(description='Short description')  

# Add binary argument with default value
parser.add_argument('-v','--verbose', action='store_true',default=False,help='print informations')

# Add required argument
parser.add_argument('task',help='task to perform',default='')
# Add required input file
parser.add_argument('infile', help='SRT file', metavar='INPUT_FILE', nargs='?', type=argparse.FileType('r'), default=sys.stdin)
# Add required output file
parser.add_argument('outfile', nargs='?', type=argparse.FileType('w'), default=sys.stdout)

# Add positional argument (list)
# positional argument  = the program  should know  what to do  with this
# argument, solely based on where it appears (e.g., the copy command
# 'cp SRC DEST')
parser.add_argument('list', metavar='filter', type=str, nargs='*', help='list of string')
# metavar: name of the variable in help
# nargs:
#   1 : 1 argument
#   + : 1 or more
#   ? : 0 or 1 (default if 0)
#   * : any number
          
# Add choice argument
parser.add_argument('move', choices=['rock', 'paper', 'scissors'])
# note no default possible
              
# Add subparser for command surch as svn
subparsers=parser.add_subparsers()
parser_foo = subparsers.add_parser('foo')
parser_bar = subparsers.add_parser('bar')


# parse
args = parser.parse_args()  
                

# list set values
print "Values in args:"
for name in args.__dict__:
    print name,' :\t',args.__dict__[name]

String formatting

Interpolation with %

Equivalent to 'sprintf':

# % operator
text = "%d little pigs come out or I'll %s and %s and %s" % \
       (3, 'huff', 'puff', 'blow down')
print(text)

# Naming argument (e.g., for reuse)
text = 'Argument %(test)s reuse: %(test)' % {'test' : 'ARG'}
print(text)

# BEWARE (tuple)
name = (1, 2, 3)
# text = "Foo bar: %s" % name # TypeError
text = "Foo bar: %s" % (name,) # Correct (but ugly)
print(text)

Method: format

text = "{} little pigs come out or I'll {} and {} and {}".format(3, 'huff', 'puff', 'blow down')
print(text)

# Naming argument (e.g., for reuse)
text = 'Argument {test} reuse: {test}'.format(test='ARG')
print(text)

# Misc
# Positional reference: 
# "{} {}" is equivalent to "{0} {1}"
# but "{2} {1} {0}" will not format the same as "{0} {1} {2}"
"Weight in tons {0.weight}"      # 'weight' attribute of first positional arg
"Units destroyed: {players[0]}" # First element of keyword argument 'players'.

Pip

Uninstall all user installed libraries

pip freeze --user | xargs pip uninstall -y

Matplotlib without font 3 in PDF

This is a pay every time i have to validate the PDF for a neurips submission! You just need to execute this before saveing the file:

import matplotlib
matplotlib.rcParams['pdf.fonttype'] = 42
matplotlib.rcParams['ps.fonttype'] = 42