Python notes

From raju

Contents

Dummy

Python related pages on this wiki

Utility functions

print command before executing it

command line arguments

Code Templates

Template code for regression testing a Python3 application

Check pep8 conformance via unittest

Functionality testing of python scripts

using printf

    indent = "  "
    printf("%s average = %.4g" %(indent, df['foo'].mean()) )
    printf("%s N = %d" %(indent, df.shape[0]) )
    

See also:-

where is a module imported from

    import imp
    imp.find_module("foo")
    

string interpolation using format()

    >>> a = 'kama'; b = 'raju'; c = "foo_{alpha}_bar_{beta}.txt".format(alpha=a, beta=b)
    >>> a
    'kama'
    >>> b
    'raju'
    >>> c
    'foo_kama_bar_raju.txt'
    

string interpolation using locals() and format()

def buildString(user, name = 'john', age=22):
    userId = user.getUserId()
    return "Name: {name}, age: {age}, userid:{userId}".format(**locals())

Tested it using Python 3.4.3

Advantage:

  • This is better than
    return "Name: {name}, age: {age}, userid:{userId}".format(name=name, age=age, userId=userId)

which is ugly.

Disadvantage:

  • Do not use it when the format string is user supplied since then it would open up access to every local variable making the application susceptible to injection attacks.

Ref:-

expand %s in a string with variable values

    >>> a = '%s:micro soft, %s:google'
    >>> b = 'MSFT'; c = 'GOOG';
    >>> a
    '%s:micro soft, %s:google'
    >>> b
    'MSFT'
    >>> c
    'GOOG'
    >>> d = a % (b, c)
    >>> d
    'MSFT:micro soft, GOOG:google'
    


string construction

Use a tuple while constructing a string with multiple placeholders

    >>> 'select * from table_name where column in (%s,%s)' % ('val1','val2')
    'select * from table_name where column in (val1,val2)'
    

Using an array will give an error

    >>> 'select * from table_name where column in (%s,%s)' % ['val1','val2']
    Traceback (most recent call last):
    
      File "<ipython-input-66-xxxxxxxxxxxx>", line 1, in <module>
        'select * from table_name where column in (%s,%s)' % ['val1','val2']
    
    TypeError: not enough arguments for format string
    

Manipulating dates

get current time as YYYYMMDD_HHMMSS

    from datetime import datetime
    datetime.now().strftime("%Y%m%d_%H%M%S")
    

iterate over a range of dates

To iterate on all days including weekends

    for dt in pd.date_range('20160226', '20160303'):
      print(dt.strftime('%Y%m%d'))
    
    20160226
    20160227
    20160228
    20160229
    20160301
    20160302
    20160303
    

To iterate only on weekdays

    for dt in pd.date_range('20160226', '20160303'):
      if (dt.weekday() < 5):
        print(dt.strftime('%Y%m%d'))
    
    20160226
    20160229
    20160301
    20160302
    20160303
    

To iterate in reverse chronological order

    for dt in pd.date_range('20160226', '20160303')[::-1]:
      if (dt.weekday() < 5):
        print(dt.strftime('%Y%m%d'))
    
    20160303
    20160302
    20160301
    20160229
    20160226
    

To experiment with just one date element

    >>> a = pd.date_range('20160226', '20160303')
    >>> a[0].strftime('%Y%m%d')
    '20160226'
    

To create a pandas.tslib.Timestamp variable

    >>> from datetime import datetime
    >>> for dt in pd.date_range('20160929', '20160930')[::-1]:
    ...     dt == pd.Timestamp(datetime(2016, 9, 29))
    ...
    False
    True
    
    >>> type(pd.Timestamp(datetime(2016, 9, 29)))
    <class 'pandas.tslib.Timestamp'>
    
    >>> for dt in pd.date_range('20160929', '20160930')[::-1]:
    ...     type(dt)
    ...
    <class 'pandas.tslib.Timestamp'>
    <class 'pandas.tslib.Timestamp'>
    

tags | convert output from pandas data_range function to YYYYMMDD, date_range reverse order

Ref:-

Convert_YYYYMMDD_to_YYYY-MM-DD

get the beginning and end dates of a month

To get the first date

    >>> from datetime import datetime
    >>> a = datetime.strptime('20160905', '%Y%m%d')
    >>> a
    datetime.datetime(2016, 9, 5, 0, 0)
    >>> b = a.replace(day=1)
    >>> b
    datetime.datetime(2016, 9, 1, 0, 0)
    >>> a
    datetime.datetime(2016, 9, 5, 0, 0)
    

To get the last date

    >>> from datetime import datetime
    >>> import calendar
    >>> a = datetime.strptime('20160905', '%Y%m%d')
    >>> a
    datetime.datetime(2016, 9, 5, 0, 0)
    >>> b = a.replace(day = calendar.monthrange(a.year, a.month)[1])
    >>> b
    datetime.datetime(2016, 9, 30, 0, 0)
    >>> a
    datetime.datetime(2016, 9, 5, 0, 0)
    

Ref:- search for datetime.replace in https://docs.python.org/2/library/datetime.html

add days to date

    >>> from datetime import datetime, timedelta
    >>> print(datetime.now())
    2018-03-15 13:02:36.416505
    >>> print(datetime.now() + timedelta(days=10))
    2018-03-25 13:02:43.200587
    

Tips

configure vim

" To add the proper PEP8 indentation
au BufRead,BufNewFile *.py,*.pyw set tabstop=4
au BufRead,BufNewFile *.py,*.pyw set softtabstop=4
au BufRead,BufNewFile *.py,*.pyw set shiftwidth=4
au BufRead,BufNewFile *.py,*.pyw set textwidth=79
au BufRead,BufNewFile *.py,*.pyw set expandtab
au BufRead,BufNewFile *.py,*.pyw set autoindent
au BufRead,BufNewFile *.py,*.pyw set fileformat=unix

tested using vim 7.2

Ref:- https://realpython.com/blog/python/vim-and-python-a-match-made-in-heaven/

Dummy

Tutorials

The advantage of the pdf version is that all the content is in a single file. The advantage of html is that python code is shown with syntax highlighting.

Profiling Python

Install python on windows

documentation links

useful articles

  • http://nedbatchelder.com/text/names1.html - article on how names, values, assignment and mutability works in Python. The concepts are explained very well. Highly recommended for C/C++ programmers looking to learn Python.

set PYTHONPATH in windows

Use ';' as the separator. For example

    PYTHONPATH="DIR1;DIR2;$PYTHONPATH" python foo.py --arg1 bar
    

So it may look

    PYTHONPATH="C:\Temp\utils;$PYTHONPATH" python foo.py --file 'C:\Temp\input\bar.csv'
    

Run python from a windows script

call it foo.cmd

    @echo off
    
    :: Anaconda path
    set ANACONDA_PATH=C:\Path\to\Continuum\Anaconda2
    
    :: Report env variables.
    :: set
    
    set PATH=%ANACONDA_PATH%;%ANACONDA_PATH%\Scripts;%PATH%
    :: Report the path
    set PATH
    
    call activate myenv
    c:/Windows/System32/where.exe python
    python --version
    python -c "import pandas as pd; print(pd.__version__)"
    call deactivate
    

try-except-else

Pseudo-code to understand the try-except-else-finally structure.

    try:
        try_this(whatever) 
    except SomeException as the_exception:
        handle_SomeException(the_exception)
        # Handle a instance of SomeException or a subclass of it.
    except Exception as the_exception:
        generic_handle(the_exception)
        # Handle any other exception that inherits from Exception
        # - doesn't include GeneratorExit, KeyboardInterrupt, SystemExit
        # Avoid bare `except:`
    else: # there was no exception whatsoever
        return something() 
        # if no exception, the "something()" gets evaluated, 
        # but the return will not be executed due to the return in the 
        # finally block below.
    finally:
        # this block will execute no matter what, even if no exception,
        # after "something" is eval'd but before that value is returned
        # but even if there is an exception.
        # a return here will hijack the return functionality. e.g.:
        return True # hijacks the return in the else clause above
    

Ref:-


What is the difference between os.environ['foo'] vs os.getenv('foo')

If the variable 'foo' is not defined, os.environ['foo'] gives an exception but os.getenv('foo') returns an empty string.

If the variable is defined, both commands give its value.

    $winpty python
    Python 3.6.1 |Anaconda 4.4.0 (64-bit)| (default, May 11 2017, 13:25:24) [MSC v.1900 64 bit (AMD64)] on win32
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import os
    >>> var = 'TEMP'
    >>> var in os.environ
    True
    >>> os.getenv(var)
    'C:\\Windows\\TEMP'
    >>> os.environ[var]
    'C:\\Windows\\TEMP'
    >>> os.getenv(var) is os.environ[var]
    True
    >>> var = 'blah'
    >>> var in os.environ
    False
    >>> os.getenv(var)
    >>> os.getenv(var) is None
    True
    >>> os.environ[var]
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "C:\ProgramData\Continuum\Anaconda\envs\py36\lib\os.py", line 669, in __getitem__
        raise KeyError(key) from None
    KeyError: 'blah'
    

show an environment variable

    import os
    try:
        user_paths = os.environ['FOO'].split(os.pathsep)
    except KeyError:
        user_paths = []
    

For example, to show the PYTHONPATH in the python interpreter

    import os
    os.environ['PYTHONPATH'].split(os.pathsep)
    

Note: The separator changes depending on the OS. For example, Linux uses ':' which Windows uses ';'.

Add a directory to the module search path

    import sys
    foo_dir = "C:\\path\\to\dir"
    if foo_dir not in sys.path:
        sys.path.append(foo_dir)
    

demonstrates | import python module into a jupyter notebook

write either to file or to stdout

Approach 1:

    import os
    odir = "foo"
    ofile = os.path.join(odir, "bar.txt")
    
    with open(ofile, "w") as fh:
      print('writing', ofile)
      data_dumper(data, fh)
    
    ...
    
    def data_dumper(data, fh)
       fh.write(data)
       ...
    

To print data onto stdout

    import sys
    data_dumper(data, sys.stdout)
    

See also: http://stackoverflow.com/questions/17602878/how-to-handle-both-with-open-and-sys-stdout-nicely

Approach 2:

    def dump_foo(arg1, arg2, ..., param={})
        import sys
        # do something
      
        # dump to a file if some parameters are specified otherwise use stdout
        if (('out_dir' in param) & ('out_file' in param)):
            out_dir = param['out_dir']
            if not os.path.exists(out_dir):
                os.makedirs(out_dir, exist_ok=True)
            out_file = os.path.join(out_dir, param['out_file'])
            fh = open(out_file, "w")
            print('writing', out_file)
        else:
             fh = sys.stdout;
    
        fh.write("I am in dump_foo()\n")
    
        if fh is not sys.stdout:
            fh.close()
    

append to a file

This is useful when writing logs. For example

    #! /usr/bin/env python3
    import os
    from datetime import datetime
    a = datetime.now().strftime("%Y%m%d_%H%M%S")
    out_dir = '/path/to/logdir'
    if not os.path.exists(out_dir):
        os.makedirs(out_dir, exist_ok=True)
    out_file_name = "log.txt"
    out_file_path = os.path.join(out_dir, out_file_name)
    with open(out_file_path, 'a') as fh:
        fh.write(a + "\n")
    

frequently used functions

Use case function Additional notes
get current directory cwd = os.getcwd()
change working directory os.chdir(path) For example os.chdir('C:\Temp\foo')
import modules from a different directory
  • sys.path.append('/path/to/dir') - directory searched last
  • sys.path.insert(0, '/path/to/dir') - directory searched first
Add parent directory to the module search path sys.path.append('..') Module Search Path
Get the PATH
  • os.getenv('PATH')
  • os.getenv('PATH').split(os.pathsep)

frequently used pdb commands

    until NNN - continue until line NNN
    

script to plot a column of data from command line

See https://gitlab.com/d3k2mk7/rutils/blob/master/python3/plot_utils/plot_by_index.py

raw strings and normpath

    a = ['test_cases\test1.csv', r'test_cases\test1.csv', 'test_cases/test1.csv', r'test_cases/test1.csv']
    a
    Out[86]: 
    ['test_cases\test1.csv',
     'test_cases\\test1.csv',
     'test_cases/test1.csv',
     'test_cases/test1.csv']
    
    b = [os.path.normpath(p) for p in a]
    b
    Out[88]: 
    ['test_cases\test1.csv',
     'test_cases\\test1.csv',
     'test_cases\\test1.csv',
     'test_cases\\test1.csv']
    
    c = [eval("r'%s'" %p) for p in a]
    c
    Out[94]: 
    ['test_cases\test1.csv',
     'test_cases\\test1.csv',
     'test_cases/test1.csv',
     'test_cases/test1.csv']
    
    d = [p.encode('string_escape') for p in a]
    d
    Out[96]: 
    ['test_cases\\test1.csv',
     'test_cases\\\\test1.csv',
     'test_cases/test1.csv',
     'test_cases/test1.csv']
    

random links

useful links

programming notes

script execution

To execute a python script in the interpreter

    exec(open('myscript.py').read(), globals())
    

But this does not allow to pass any arguments to the script. For that use

    import sys
    import subprocess
    subprocess.call([sys.executable, 'foo.py', arg1])
    

For windows, the filepath can be specified as 'C:/path/to/foo.py'

check if a file exists

    import os
    if not os.path.isfile(ofile):
        print("File", ofile, "does not exist")
    

Ref:- http://stackoverflow.com/questions/82831/how-do-i-check-whether-a-file-exists-using-python

Check if a list is empty

    >>> a = []
    >>> if (not a):
    ...     print("List is empty")
    ... 
    List is empty
    

check if a key is exists in a dictionary

    if 'key1' in dict:
      print "blah"
    else:
      print "boo"
    

Ref: http://stackoverflow.com/questions/1602934/check-if-a-given-key-already-exists-in-a-dictionary

check if something is not None

In:

    a=None
    
    if (a is not None):
        print("kama")
    else:
        print("raju")
        
    

Out:

    raju
    

In:

    a=2
    
    if (a is not None):
        print("kama")
    else:
        print("raju")
        
    

Out:

    kama
    

check if a list is sorted

     % cat is_list_sorted.py
    def is_list_sorted(l):
        return all( [l[i] < l[i+1] for i in range(len(l)-1)] )
    
    a = [10,1,8,2,5,3,7,9,6,4]
    print(is_list_sorted(a))
    
    b = sorted(a)
    print(is_list_sorted(b))
    
     % python3 -u is_list_sorted.py
    False
    True
    

Ref:- https://stackoverflow.com/questions/3755136/pythonic-way-to-check-if-a-list-is-sorted-or-not

reload a module

For python 3

    from importlib import reload
    reload(foo)
    

For python 2

    reload(foo)
    

Note:- if you are importing functions from a module using

    from foo import *
    

then reload(foo) will throw a NameError.

    NameError: name 'foo' is not defined
    

This happens because there is no object bound to foo. As a work around, I do

    from foo import *
    import foo
    reload(foo)
    

See https://stackoverflow.com/questions/13455836/nameerror-when-using-reload for more details.

which python executable am I running?

    import sys
    print(sys.executable)
    

For example

    $python
    Python 3.6.1 |Anaconda 4.4.0 (64-bit)| (default, May 11 2017, 13:25:24) [MSC v.1900 64 bit (AMD64)] on win32
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import sys
    >>> print(sys.executable)
    C:\ProgramData\Continuum\Anaconda\envs\py36\python.exe
    

split string and trim white space

    s = "blah, lots  ,  of ,  spaces, here "
    [x.strip() for x in s.split(',')]
    
    Out[18]: ['blah', 'lots', 'of', 'spaces', 'here']
    

Ref:- http://stackoverflow.com/questions/4071396/split-by-comma-and-strip-whitespace-in-python


convert two lists into a dictionary

Use dict with zip to construct a dictionary from two lists. The number of elements in dictionary will be the minimum of the number of elements in each list. The extra elements will be discarded.

     % python3          
    Python 3.5.2+ (default, Sep 22 2016, 12:18:14) 
    [GCC 6.2.0 20160914] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    
    >>> a = [1, 2, 3]; b = ['a', 'b', 'c']; c = dict(zip(a, b)); print(a); print(b); print(c)
    [1, 2, 3]
    ['a', 'b', 'c']
    {1: 'a', 2: 'b', 3: 'c'}
    
    >>> a = [1, 2, 3, 4]; b = ['a', 'b', 'c']; c = dict(zip(a, b)); print(a); print(b); print(c)
    [1, 2, 3, 4]
    ['a', 'b', 'c']
    {1: 'a', 2: 'b', 3: 'c'}
    
    >>> a = [1, 2, 3]; b = ['a', 'b', 'c', 'd']; c = dict(zip(a, b)); print(a); print(b); print(c)
    [1, 2, 3]
    ['a', 'b', 'c', 'd']
    {1: 'a', 2: 'b', 3: 'c'}
    


debug from command line

To pass command line arguments and invoke the debugger directly

    python -u -m pdb mymodule.py --arg1_key arg1_val --arg2_key arg2_val
    

To do it inside python interpreter

    import pdb
    import mymodule
    pdb.run('mymodule.myfunc()')
    

Ref:-

tags | Debugging python programs from the command line

step through a python script

tags | debug Add the following lines at the beginning of the script

    import pdb
    pdb.set_trace()
    

Ref:- https://pythonconquerstheuniverse.wordpress.com/2009/09/10/debugging-in-python/

returning values

One way to return multiple values in a function is to use a dictionary.

    return {'key1':value1,'key2':value2,'key3':value3}
    

or

    result = {'key1':value1}
    result['key2'] = value2
    result['key3'] = value3
    return(result)
    

Ref:-

if else if syntax

    if x < 0:
        x = 0
        print('Negative changed to zero')
    elif x == 0:
        print('Zero')
    elif x == 1:
        print('Single')
    else:
        print('More')
    

check for missing data

    if (x is not None and x > 0):
        print('x is not none and positive')
    

logical operators

    x or y, x and y, not x
    

Example

        if (a < 1 or a > 100):
            print('a is not in [1,100]')
    

Ref:- https://docs.python.org/3/library/stdtypes.html

timing a function

    start_time = time.time()
    call_function_of_interest()
    end_time = time.time()
    print('time elapsed = ', end_time - start_time)
    

For python2

    print 'time elapsed = ', end_time - start_time
    print 'took %.f sec to finish' %(end_time - start_time)
    

tags | time a function, tic toc

Remove duplicates

To remove duplicates from a column of a dataframe

    unique_id = df['id'].drop_duplicates()
    

Ref:- http://pandas.pydata.org/pandas-docs/version/0.17.0/generated/pandas.Series.drop_duplicates.html

tags | unique

Replace newline characters with space

    % python3
    Python 3.4.3 (default, Mar  3 2015, 15:56:43)
    [GCC 4.4.7 20120313 (Red Hat 4.4.7-4)] on linux
    
    >>> a = '''kama
    ... raju'''
    >>> a
    'kama\nraju'
    >>> a.replace("\n", " ")
    'kama raju'
    

overlay one column with another

Say you have two columns 'a' and 'b' in a dataframe. To copy the values from column 'a' to column 'b' whenever the values in column 'b' are missing.

    df.loc[ pd.isnull(df['b']), 'b'] = \
      df.loc[ pd.isnull(df['b']), 'a']
    

tags | pandas update data based on a condition, indexing and selecting data, find and replace values in a python dataframe, replace values if null, assign values by index, search and replace data based on another column using python pandas, example on using loc, python pandas boolean array filter

specify the path of a file

To specify the path of a file relative to the location of the module

    fname = os.path.join(os.path.dirname(__file__), 'foo.txt')
    

__file__ gives the path of current module.

show all variables in python interpreter

use dir()

    >>> dir()
    ['__builtins__', '__doc__', '__loader__', '__name__', '__package__', '__spec__']
    >>> import pandas as pd
    >>> dir()
    ['__builtins__', '__doc__', '__loader__', '__name__', '__package__', '__spec__', 'pd']
    >>> df = pd.DataFrame([[1, 2], [3, 4]], columns=list('AB'))
    >>> dir()
    ['__builtins__', '__doc__', '__loader__', '__name__', '__package__', '__spec__', 'df', 'pd']
    >>> df
       A  B
    0  1  2
    1  3  4
    

sql query string

    sql_query = '''
    select *
      from alpha..beta
     where foo in ('{foo}')
       and bar in ('{boo}')
    '''
    res = db_query(sql_query.format(foo = "', '".join(foo),
                                    bar = "', '".join(bar))
                  )
    
    

where db_query is a custom function to query the database.

connecting to databases

Using pyodbc to connect to MSSQL

        import pyodbc
        import pandas as pd
        # declare user, password, server, database
        conn_str = 'DRIVER={SQL Server};SERVER=' + server + ';DATABASE=' + database
        if user is not None and password is not None:
            conn_str += ';UID=' + user + ';PWD=' + password
        else:
            conn_str += ';Trusted_Connection=yes'
        connection = pyodbc.connect(conn_str)
        data = pd.io.sql.read_sql(<SQL goes here>, connection)
    

Using SQLAlchemy

    from urllib import quote_plus as urlquote
    
    import sqlalchemy as sqla
    import pandas as pd
    
    def create_engine(conn_str):
        url = sqla.engine.url.make_url(conn_str)
        if url.drivername == 'oracle+cx_oracle':
            return sqla.create_engine(url, arraysize=100000)
        else:
            return sqla.create_engine(url)
    

Then for MSSQL

    # declare user, password, server, database
    driver = 'ODBC Driver 11 for SQL Server'
    
    if user is not None and password is not None:
        conn_str = "mssql+pyodbc://{}:{}@{}/{}?driver={}".format(
            user, urlquote(password), server, database, urlquote(driver)
        )
    else:
        # use windows authentication
        conn_str = "mssql+pyodbc://{}/{}?driver={};trusted_connection=yes".format(
            server, database, urlquote(driver)
        )
    engine = create_engine(conn_string)
    data = pd.read_sql_query('<SQL goes here>', engine)
    

and for Oracle

    # declare user, password, server
    conn_str = "oracle+cx_oracle://{}:{}@{}".format(
        user, urlquote(password), server
    )
    engine = create_engine(conn_string)
    data = pd.read_sql_query('<SQL goes here>', engine)
    

writing an if else condition in list comprehension

Syntax

    [ a if C else b for i in items]
    

is equivalent to

    for i in items:
      if (C):
        a
      else:
        b
    

For example, to figure out if a package is installed one can do

    import pandas as pd
    import apt
    cache = apt.Cache()
    
    a = pd.DataFrame({'package': ['python-apt', 'foo', 'dpkg', '0xffff']})
    status = [
        True if ((pkg in cache) and cache[pkg].is_installed) else False
        for pkg in a['package']
    ]
    
    print(status)
    

which will show

    [True, False, True, False]
    

where python-apt, dpkg are installed on the system, '0xffff' is a valid package but not installed and 'foo' is not a valid package name.

Ref: http://stackoverflow.com/questions/4406389/if-else-in-a-list-comprehension

Stackoverflow links

get the username

    >>> import getpass
    >>> getpass.getuser()
    'rajulocal'
    

exit with error

One way is to throw an exception. See http://stackoverflow.com/questions/2052390/manually-raising-throwing-an-exception-in-python for more details.

args and kwargs

args = arguments kwargs = keyword arguments

    def f(*args, **kwargs):
      print('args: ', args, ' kwargs: ', kwargs)
    
    >>> f('a')
    args:  ('a',)  kwargs:  {}
    >>> f(ar='a')
    args:  ()  kwargs:  {'ar': 'a'}
    >>> f(1,2,param=3)
    args:  (1, 2)  kwargs:  {'param': 3}
    
    >>> f(1,[2,3],param=3)
    args:  (1, [2, 3])  kwargs:  {'param': 3}
    
    >>> f(1, [2,3], [4,5,6], param=3, first='kama', last='raju')
    args:  (1, [2, 3], [4, 5, 6])  kwargs:  {'param': 3, 'last': 'raju', 'first': 'kama'}
    

See also: http://stackoverflow.com/questions/682504/what-is-a-clean-pythonic-way-to-have-multiple-constructors-in-python?noredirect=1&lq=1


using kargs

Sample usage

    def foo(alpha, beta, **kwargs):
        debug = kwargs.get('debug', False)
    

multiline statements

    df = pd.read_csv(fname)\
    .pipe(func_foo)\
    .pipe(func_bar)
    

conditional operator

tags | conditional expressions, ternary

    a if condition else b
    

calling python script from bash

print stack trace

To print stack trace from within the program

    import traceback
    print 'printing stack'
    traceback.print_stack()
    

Ref:- https://docs.python.org/2/library/traceback.html

parse config files

Example 1:

     % cat config.ini
    [COUNTRIES]
    DENY = AG AL
    ALLOW = IN US
    
     % cat config.py
    import configparser
    import os
    
    Config = configparser.ConfigParser()
    Config.read(os.path.join(os.path.dirname(__file__), 'config.ini'))
    

To use it

     % python3
    Python 3.5.3 (default, Jan 19 2017, 14:11:04)
    [GCC 6.3.0 20170118] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    >>> from config import Config
    >>> Config['COUNTRIES']['DENY']
    'AG AL'
    >>> Config['COUNTRIES']['ALLOW']
    'IN US'
    

What is the difference between <> and !=

In python 3, there is no <> operator.

In python 2, they both mean the same. != is preferred; <> is obsolescent.

Ref:-

Porting windows code to Linux

Things to look out for when porting python code from windows to Linux

  • use os.path.sep instead of "\\" for parsing directories in a filepath
  • use os.path.join when constructing file name. For example instead of dir_name + "\\" + file_name, use os.path.join(dir_name, file_name)
  • exception during reset or similar

numpy

Frequently used numpy functions

use case solution
average np.average
sum np.sum

Challenges

Rotate matrix anti clockwise

Given a 2-D array

    [[1,2,3,4],
     [5,6,7,8],
     [9,10,11,12]]
    

convert it to

    [[4,8,12],
     [3,7,11],
     [2,6,10],
     [1,5,9]]
    
    >>> a = [[1,2,3,4], [5,6,7,8], [9,10,11,12]]
    >>> a
    [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]
    >>> b = list(zip(*a))[::-1]
    >>> b
    [(4, 8, 12), (3, 7, 11), (2, 6, 10), (1, 5, 9)]
    >>> c = [list(elem) for elem in b]
    >>> c
    [[4, 8, 12], [3, 7, 11], [2, 6, 10], [1, 5, 9]]
    

Rotate matrix clockwise

Given a 2-D array

    [[1,2,3,4],
     [5,6,7,8],
     [9,10,11,12]]
    

convert it to

    [[9,5,1],
     [10,6,2],
     [11,7,3],
     [12,8,4]]
    
    >>> a = [[1,2,3,4], [5,6,7,8], [9,10,11,12]]
    >>> a
    [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]
    >>> b = list(zip(*a[::-1]))
    >>> b
    [(9, 5, 1), (10, 6, 2), (11, 7, 3), (12, 8, 4)]
    >>> c = [list(elem) for elem in b]
    >>> c
    [[9, 5, 1], [10, 6, 2], [11, 7, 3], [12, 8, 4]]
    

index of an item in a sorted list

Use bisect module to find out where an item can be inserted in a sorted list. The return value of bisect is suitable for use as the first parameter to list.insert() assuming that original list is already sorted.

    >>> a = [10,1,8,2,5,3,7,9,6,4]
    >>> a
    [10, 1, 8, 2, 5, 3, 7, 9, 6, 4]
    >>> a.sort()
    >>> a
    [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
    >>> x = 5.5
    >>> from bisect import bisect
    >>> bisect(a,x)
    5
    >>> a.insert( bisect(a,x), x)
    >>> a
    [1, 2, 3, 4, 5, 5.5, 6, 7, 8, 9, 10]
    

Instead of doing bisect and insert, you can also insort which does both.

    >>> a = [10,1,8,2,5,3,7,9,6,4]
    >>> a
    [10, 1, 8, 2, 5, 3, 7, 9, 6, 4]
    >>> a.sort()
    >>> a
    [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
    >>> x = 5.5
    >>> from bisect import insort
    >>> insort(a,x)
    >>> a
    [1, 2, 3, 4, 5, 5.5, 6, 7, 8, 9, 10]
    

Ref:- https://docs.python.org/3/library/bisect.html