Python notes

From raju



Python related pages on this wiki


no module named error when importing from subdirectories

  • Make sure that each directory contains

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

string interpolation using format()

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

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


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

which is ugly.


  • 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.


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
    >>> c
    >>> 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

pep8 write long strings

        user_agent = 'Mozilla/5.0 (Windows NT 6.1; Win64; x64) ' \
            'AppleWebKit/537.36 (KHTML, like Gecko) ' \
            'Chrome/66.0.3359.139 Safari/537.36'


print objects

    class SimpleRepr(object):
        A mixin implementing a simple __repr__.
        It gives the class name, the (shortened) id, and all of the attributes.
        def __repr__(self):
            return "<{klass} @{id:x} {attrs}>".format(
                id=id(self) & 0xFFFFFF,
                attrs=" ".join("{}={!r}".format(k, v) for k, v in self.__dict__.items()),

Use it as follows

    class DataWrapper(SimpleRepr):
        def __init__(self, args):
            args_dict = vars(args)
            for (k,v) in args_dict.items():
                setattr(self, k, v)
    if __name__ == '__main__':
        # parse arguments using argparser
        data_wrapper = DataWrapper(parsed_args)


Manipulating dates

get today's date as YYYYMMDD

    from datetime import datetime'%Y%m%d')


    $ ipython
    In [1]: from datetime import datetime
    In [2]:
    Out[2]: datetime.datetime(2018, 10, 1, 16, 25, 43, 104336)
    In [3]:'%Y%m%d')
    Out[3]: '20181001'

get current time as YYYYMMDD_HHMMSS

    from datetime import datetime"%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'):

To iterate only on weekdays

    for dt in pd.date_range('20160226', '20160303'):
      if (dt.weekday() < 5):

To iterate in reverse chronological order

    for dt in pd.date_range('20160226', '20160303')[::-1]:
      if (dt.weekday() < 5):

To experiment with just one date element

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

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))
    >>> 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



Convert excel days to datetime

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

add days to date

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


If date is available as year, month, day

    In [1]: from datetime import datetime
       ...: (datetime(2019, 8, 17) - datetime(1899, 12, 30)).days
    Out[1]: 43694

If date is available as a YYYYMMDD string

    In [1]: from datetime import datetime
       ...: (datetime.strptime('20180602', '%Y%m%d') - datetime.strptime('20180418', '%Y%m%d')).days
    Out[1]: 45

In practice

    >>> d1 = '20180418'
    >>> d2 = '20180602'
    >>> from datetime import datetime
    >>> fmt = '%Y%m%d'
    >>> dt1 = datetime.strptime(d1, fmt)
    >>> dt2 = datetime.strptime(d2, fmt)
    >>> delta = dt2 - dt1
    >>> print delta
    45 days, 0:00:00
    >>> print delta.days

extract YYYYMMDD from a string

tags | extract date from filename, using re.split

Say we have files of the form foo_YYYYMMDD.csv, the idea here is extract YYYYMMDD from those file names.


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


Copying files

recursively copy files and overwrite if necessary

tags | copying files, copytree overwrite

tags | preserve timestamp when copying files

Use distutils.dir_util.copytree

Sample code snippet

    from distutils.dir_util import copy_tree
    if os.path.isdir(src):
        copy_tree(src, dst)

This will recursively copy whatever is in src to dst (ex:- src/foo to dst/foo, src/foo/bar/baz.txt to dst/foo/bar/baz.txt etc.,)

If same file exists in both, the one in src overwrites the one in dst.

If a file exists only in dst, but not in src, it is not touched.


  • src has to be a directory
  • dst will be created if it does not already exist
  • all child directories under dst will be created as needed
  • dst can be an existing directory but not an existing file
  • timestamps are preserved by default


Why not shutil.copytree?

  • shutil.copytree throws an error if dst directory already exists.
    WindowsError: [Error 183] Cannot create a file when that file already exists: '/path/to/dst'

distutils.dir_util.copy_tree does not care if that is the case. It will simply overwrite the files inside it.

copying files

To copy a single file, you can use shutil.copyfile, shutil.copy(), shtuil.copy2()


    shutil.copyfile(src, dst, *, follow_symlinks=True)
    shutil.copy(src, dst, *, follow_symlinks=True)
    shutil.copy2(src, dst, *, follow_symlinks=True)


destination will be overwritten if it already exists. dst has to be a file and cannot be a directory.

For example

    shutil.copyfile('C:/Users/kkusuman/x/x1/junk1.txt', 'C:/Users/kkusuman/x/x1/junk2.txt')
    shutil.copyfile('C:\\Users\\kkusuman\\x\\x1\\junk1.txt', 'C:\\Users\kkusuman\\x\\x1\\junk2.txt')


dst can either be a file or a directory. It copies the data and file's permission mode. Other metadata such as file's creation and modification times are not preserved.


same as shutil.copy() except that it preserves all metadata.


Delete files

deleting files

    def remove_file(fname):
        if os.path.isfile(fname):
            except Exception as exc:
                print exc

delete directory recursively

    import os
    from distutils.dir_util import remove_tree
    if os.path.isdir(fname):

disttuils.dir_util.remove_tree works only on a directory and the directory should exist. Otherwise, it will throw an exception.

Reading files

read file into a list

    $ cat great.txt
    In [2]: file = 'great.txt'
       ...: with open(file) as fh:
       ...:     contents = [line.rstrip() for line in fh]
       ...: print(contents)
    ['kamaraju', 'kamaraj', 'kamara', 'kamar', 'kama', 'kam', 'ka', 'k']
    In [3]: len(contents)
    Out[3]: 8


If you use readlines(), there will be a new line character at the end.

    In [4]: file = 'great.txt'
       ...: with open(file) as fh:
       ...:     contents = fh.readlines()
       ...: print(contents)
    ['kamaraju\n', 'kamaraj\n', 'kamara\n', 'kamar\n', 'kama\n', 'kam\n', 'ka\n', 'k\n']


first common element

tags | find the first element common to two lists

Q: Given two lists x, y find the first element in x that is present in y?

A: This does it in O(N) time with a set (hash map)

next((i for i in x if i in set_y), None)

Sample run:

    $ ipython
    In [1]: x = [1,2,3,4]
       ...: y = [2,5,6,7]
       ...: set_y = set(y)
       ...: c = next((i for i in x if i in set_y), None)
       ...: print(c)
    In [2]: x = [1,3,4,5]
       ...: c = next((i for i in x if i in set_y), None)
       ...: print(c)
    In [3]: x = [1,3,4,8]
       ...: c = next((i for i in x if i in set_y), None)
       ...: print(c)




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

  • - 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 --arg1 bar

So it may look

    PYTHONPATH="C:\Temp\utils;$PYTHONPATH" python --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
    :: 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


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

    except SomeException as the_exception:
        # Handle a instance of SomeException or a subclass of it.
    except Exception as 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.
        # 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


os.path.isfile vs os.path.exists

  • os.path.isfile - is it an existing regular file?
  • os.path.exist - does it exist?

os.path.isfile also checks for existence. So no need to use both os.path.isfile and os.path.exists.

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
    >>> os.getenv(var)
    >>> os.environ[var]
    >>> os.getenv(var) is os.environ[var]
    >>> var = 'blah'
    >>> var in os.environ
    >>> os.getenv(var)
    >>> os.getenv(var) is None
    >>> os.environ[var]
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "C:\ProgramData\Continuum\Anaconda\envs\py36\lib\", line 669, in __getitem__
        raise KeyError(key) from None
    KeyError: 'blah'

rename directories

tags | rename directory, move a directory only if it exists, apply a function to each row in a pandas dataframe

    to_rename = pd.DataFrame({
        'src': ['/path/to/src1', '/path/to/src2', '/path/to/src3', '/path/to/src4'],
        'dst': ['/path/to/dst1', '/path/to/dst2', '/path/to/dst3', '/path/to/dst4']
    to_rename.apply(lambda row: os.rename(row['src'], row['dst']) if os.path.exists(row['src']) else None, axis=1)

show an environment variable

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

For example, to show the PYTHONPATH in the python interpreter

    import os

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

check the python version

    >>> import sys
    >>> print(sys.version)
    3.6.6 |Anaconda custom (64-bit)| (default, Jun 28 2018, 11:27:44) [MSC v.1900 64 bit (AMD64)]

Add a directory to the module search path

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

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)

To print data onto stdout

    import sys
    data_dumper(data, sys.stdout)

See also:

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)
             fh = sys.stdout;
        fh.write("I am in dump_foo()\n")
        if fh is not sys.stdout:

append to a file

This is useful when writing logs. For example

    #! /usr/bin/env python3
    import os
    from datetime import datetime
    a ="%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


raw strings and normpath

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

random links

useful links


The image can also be accessed via

Virtual Environments

The PyPI version of virtualenv works in most environments. As of Python 3.3, the venv virtual environment module is included as part of the standard library. However, some problems with venv have been reported on Ubuntu. Since virtualenv works with Python 3.6 (and as far back as Python 2.6) and on Ubuntu, use virtualenv.

Setup virtual environment

Using pip in Linux

    pip install -U virtualenv
    virtualenv -p /path/to/a/python.exe /path/to/env_name
    source /path/to/env_name/bin/activate
    # do your work

Uisng Python in Linux

    python3.6 -m pip install -U virtualenv
    python3.6 -m virtualenv env_name
    source env_name/bin/activate
    # do your work

Using pip in Windows

    pip install -U virtualenv
    virtualenv -p /path/to/a/python.exe /path/to/env_name
    # do your work

Using Python in Windows

    python3.6 -m pip install -U virtualenv
    python3.6 -m virtualenv env_name
    # do your work

programming notes

script execution

To execute a python script in the interpreter

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

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

    import sys
    import subprocess[sys.executable, '', arg1])

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

check if a file exists

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


check if a key is exists in a dictionary

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


check if two values are not none

    if all(v is not None for v in [A, B, C, D, E]):

check if something is not None


    if (a is not None):




    if (a is not None):



check if a list is sorted

     % cat
    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]
    b = sorted(a)
     % python3 -u


check if a string can be converted to float

    def is_float_by_except(s):
            return True
        except ValueError:
            return False

lambda if


Task:- Copy files in a dataframe column to a destination folder.

We need to check if the file exists before copying it since otherwise copy_file will throw an error.

    from distutils.file_util import copy_file
    import os
    df['file'].apply(lambda x: copy_file(x, out_dir) if os.path.isfile(x) else None)

sort run_1 run_2 run_10

tags | pass custom function to sorted

The task here is to sort a list of strings of form 'run_N' on N where N is an integer. For example, if we have ['run_9', 'run_2', 'run_10', 'run_1'], the output should be ['run_1', 'run_2', 'run_9', 'run_10']

The default string sorting using the sorted function will not work since 'run_10' is smaller than 'run_2' in pure string comparison.

    $ ipython
    In [1]: a = ['run_9', 'run_2', 'run_10', 'run_1']
    In [2]: sorted(a)
    Out[2]: ['run_1', 'run_10', 'run_2', 'run_9']

Instead one has to extract the integer, N, and use it as the key.

    In [1]: a = ['run_9', 'run_2', 'run_10', 'run_1']
    In [2]: import re
    In [3]: pattern = 'run_' + '(\d+)$'
    In [4]: sorted(a, key=lambda x: int(, x).group(1)))
    Out[4]: ['run_1', 'run_2', 'run_9', 'run_10']

extract something out of a string

     % python3
    Python 3.5.3 (default, Sep 27 2018, 17:25:39)
    [GCC 6.3.0 20170516] on linux
    >>> import re
    >>> a = "GeoIP Country Edition: US, United States"
    >>> b = "blah blah"
    >>> pattern = "GeoIP Country Edition: (\w+), "
    >>> re.match(pattern, a)
    <_sre.SRE_Match object; span=(0, 27), match='GeoIP Country Edition: US, '>
    >>> re.match(pattern, b)
    >>> re.match(pattern, a) is None
    >>> re.match(pattern, b) is None
    >>>, a)
    <_sre.SRE_Match object; span=(0, 27), match='GeoIP Country Edition: US, '>
    >>>, b)
    >>>, a).group(1)
    >>>, b).group(1)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    AttributeError: 'NoneType' object has no attribute 'group'
    >>> c =, a).group(1) if re.match(pattern, a) else None
    >>> c
    >>> c =, b).group(1) if re.match(pattern, b) else None
    >>> c
    >>> c is None

reload a module

For python 3

    import importlib

For python 2


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

See for more details.

which python executable am I running?

    import sys

For example

    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)

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']


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 --arg1_key arg1_val --arg2_key arg2_val

To do it inside python interpreter

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


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


returning values

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

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


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


if else if syntax

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

check for missing data

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

using if else in a line

    >>> a = 5 if 2 > 3 else 4
    >>> a
    >>> a = 5 if 3 > 2 else 4
    >>> a

interval comparison

Python supports chained comparison such as x < y <= z. It is equivalent to x < y and y <= z. In general a op1 b op2 c ... y opN z is equivalent to a op1 b and b op2 c and ... y opN z


tags | check if a value is in between two values, multiple logical operators

logical operators

    x or y, x and y, not x


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


bitwise logic operators

    &, |, ^, and ~


    np.sum((inches > 0.5 & (inches < 1))

Parenthesis are important here because of operator precedence rules. Without the parenthesis, the expression will be evaluated as Example

    inches (> 0.5 &) < 1

Another way to express the above logic

    np.sum(~( (inches <= 0.5) | (inches >= 1) ))

sinc A AND B is same as NOT (NOT A OR NOT B)

operator precedence

timing a function

    start_time = time.time()
    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()


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
    >>> 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
            conn_str += ';Trusted_Connection=yes'
        connection = pyodbc.connect(conn_str)
        data =<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)
            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)
        # 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


    [ a if C else b for i in items]

is equivalent to

    for i in items:
      if (C):

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']

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.


Stackoverflow links

get the username

    >>> import getpass
    >>> getpass.getuser()

exit with error

One way is to throw an exception. See for more details.

exit with control d in git bash

Use ipython to exit using control d in git bash.

control d does not work with regular python. There you have to use either exit() or ctrl z <enter>.

enter multiline statements in ipython

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:

using kargs

Sample usage

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

multiline statements

    df = pd.read_csv(fname)\

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'


print exception and continue

    from traceback import format_exc
        # my code
    except Exception as exc:
        print 'caught exception while doing foo'
        print format_exc()
        print exc

print exception to log file

    from traceback import format_exc
        # my code
    except Exception as exc:
        logger.error('Caught exception while doing stuff.')

When the code is executed in pycharm and the exception is encountered, the traceback is printed in both pycharm window and in the file.

To test the above code, you can raise a simple exception using

    raise ValueError("test exception handling.")

parse config files

Example 1:

     % cat config.ini
    DENY = AG AL
     % cat
    import configparser
    import os
    Config = configparser.ConfigParser(), '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.


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

Download a webpage

version 1:-

    import requests
    def get_html(url):
        # Get html from url
        response = requests.get(url)
        return response.text

Version 2:- Same as version 1, but has user_agent hardcoded

    import requests
    def get_html(url):
        # Get html from url
        user_agent = 'Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.139 Safari/537.36'
        header = {'User-Agent': user_agent}
        response = requests.get(url, headers=header)
        return response.text

what is **{}

    Python 3.6.6 |Anaconda, Inc.| (default, Jun 28 2018, 11:27:44) [MSC v.1900 64 bit (AMD64)] on win32
    >>> def foo(**bar):
    ...   print(bar)
    >>> foo(**{'a':2})
    {'a': 2}
    >>> foo(**{'a':2, 'b':3})
    {'a': 2, 'b': 3}
    >>> foo()


using timeit

    $ python -mtimeit "l=[]"
    10000000 loops, best of 3: 0.0274 usec per loop
    $ python -mtimeit "l=list()"
    10000000 loops, best of 3: 0.125 usec per loop

pep8 recommendations

  • use 4-space indentation, and no tabs
  • 79 characters per line
  • use CamelCase for classes and lower_case_with_underscores for functions and methods


python web interpreters

share variables across different files

tags | globals, singleton, variables shared across modules

See |

See also


os.makedirs is like "mkdir -p". It will create intermediate directories if necessary.

get username in windows


returns the user that is executing the script.

string formatting in assert statements

    In [1]: assert '10' == 10, 'not equal {} blah {}'.format(10, 1)
    AssertionError                            Traceback (most recent call last)
    <ipython-input-1-31a4cefc7818> in <module>()
    ----> 1 assert '10' == 10, 'not equal {} blah {}'.format(10, 1)
    AssertionError: not equal 10 blah 1
    In [2]: assert '10' == 10, 'not equal {} blah {}'.format(10, '100')
    AssertionError                            Traceback (most recent call last)
    <ipython-input-2-8bee5a255efa> in <module>()
    ----> 1 assert '10' == 10, 'not equal {} blah {}'.format(10, '100')
    AssertionError: not equal 10 blah 100
    In [3]: assert '10' == 10, 'not equal %d blah %s' % (10, '100')
    AssertionError                            Traceback (most recent call last)
    <ipython-input-3-fa41c2fc7bb1> in <module>()
    ----> 1 assert '10' == 10, 'not equal %d blah %s' % (10, '100')
    AssertionError: not equal 10 blah 100

small experiments


    % 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.
    >>> float('inf')
    >>> type(float('inf'))
    <class 'float'>
    >>> float('inf') > 5
    >>> float('inf') > -float('inf')
    >>> float('inf') is float('inf')
    >>> float('inf') == float('inf')


Frequently used numpy functions

use case solution
average np.average
sum np.sum

OOP related

referring to class variables in methods

tags | use classmethod to access class variables

    $ cat
    class Days():
        SUN = "Sunday"
        MON = "Monday"
        TUE = "Tuesday"
        WED = "Wednesday"
        THU = "Thursday"
        FRI = "Friday"
        SAT = "Saturday"
        def weekdays(cls):
            return [cls.MON, cls.TUE, cls.WED, cls.THU, cls.FRI]
        def weekends(cls):
            return [cls.SAT, cls.SUN]

To use them in

    from foo import Days
    a = Days.SUN
    b = Days.weekdays()
    c = Days.weekends()

See also |


Rotate matrix anti clockwise

Given a 2-D array


convert it to

    >>> 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


convert it to

    >>> 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)
    >>> 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]




In , I see things like

        def get_sma(self, symbol, interval='daily', time_period=20, series_type='close'):

What do those @av decorators do? Read about this later.