python interview questions (1)

   Think in python, a nice online python book.


1. What is Python?
Python is an interpreted, interactive, object-oriented programming language. It incorporates modules, exceptions, dynamic typing, very high level dynamic data types, and classes.


2.  How can I find the methods or attributes of an object?
For an instance x of a user-defined class, dir(x) returns an alphabetized list of the names containing the instance attributes and methods and attributes defined by its class.

3. How do I convert a string to a number?
For integers, use the built-in int() type constructor, e.g. int('144') == 144. Similarly, float() converts to floating-point, e.g. float('144') == 144.0. The number 144 to the string '144', use the built-in function str().

4. What is tuple and list? How to convert between tuple and list?
A tuple is a sequence of values. The values can be any type, and they are indexed by integers, so in that respect tuples are a lot like lists. The important difference is that tuples are immutable.

Like a string, a list is a sequence of values. In a string, the values are characters; in a list, they can be any type. The values in a list are called elements or sometimes items. The list is mutable.

A dictionary is like a list, but more general. In a list, the indices have to be integers; in a dictionary they can be (almost) any type.

The function tuple(seq) converts any sequence (actually, any iterable) into a tuple. The function list(seq) converts any sequence or iterable into a list with the same items in the same order.  

5. What is numpy and scipy?
numpy is a python extension module to support efficient operation on arrays of homogeneous data.


SciPy is a set of Open Source scientific and numeric tools for Python. It currently supports special functions, integration, ordinary differential equation (ODE) solvers, gradient optimization, parallel programming tools, an expression-to-C++ compiler for fast execution, and others.

6.What scientific libraries are available in SciPy?

The full SciPy also has the following modules:

cluster

information theory functions (currently, vq and kmeans)

fftpack

fast Fourier transform module based on fftpack and fftw when available

integrate

numeric integration for bounded and unbounded ranges. ODE solvers.

interpolate

interpolation of values from a sample data set.

io

reading and writing numeric arrays, MATLAB .mat, and Matrix Market .mtx files

lib

access to the BLAS and LAPACK libraries

linalg

linear algebra and BLAS routines based on the ATLAS implementation of LAPACK

maxentropy

Support for fitting maximum entropy models, either discrete or continuous

misc

other routines that don't clearly fit anywhere else. The Python Image Library (PIL) interface is located here.

optimize

constrained and unconstrained optimization methods and root-finding algorithms

signal

signal processing (1-D and 2-D filtering, filter design, LTI systems, etc.)

sparse

Some sparse matrix support. LU factorization and solving Sparse linear systems

special

special function types (bessel, gamma, airy, etc.)

stats

statistical functions (stdev, var, mean, etc.)

weave

compilation of numeric expressions to C++ for fast execution

6. How to load an array from a text file in pyhton?

Use numpy.loadtxt. Even if your text file has header and footer lines or comments, loadtxt can almost certainly read it; it is convenient and efficient.

7. What is matplotlib and pylab?

matplotlib is a python 2D plotting library which produces publication quality figures in a variety of hard-copy formats and interactive environments across platforms.

The package pylab combines matplotlib, scipy with numpy into a single namespace.

    from pylab import *

That gets them NumPy, SciPy, and Matplotlib. A rough equivalent would be:

   from pylab import *
   from numpy import *
   rom scipy import *

plotting commands

Function	Description
acorr	plot the autocorrelation function
annotate	annotate something in the figure
arrow	add an arrow to the axes
axes	create a new axes
axhline	draw a horizontal line across axes
axvline	draw a vertical line across axes
axhspan	draw a horizontal bar across axes
axvspan	draw a vertical bar across axes
axis	set or return the current axis limits
barbs	a (wind) barb plot
bar	make a bar chart
barh	a horizontal bar chart
broken_barh	a set of horizontal bars with gaps
box	set the axes frame on/off state
boxplot	make a box and whisker plot
cla	clear current axes
clabel	label a contour plot
clf	clear a figure window
clim	adjust the color limits of the current image
close	close a figure window
colorbar	add a colorbar to the current figure
cohere	make a plot of coherence
contour	make a contour plot
contourf	make a filled contour plot
csd	make a plot of cross spectral density
delaxes	delete an axes from the current figure
draw	Force a redraw of the current figure
errorbar	make an errorbar graph
figlegend	make legend on the figure rather than the axes
figimage	make a figure image
figtext	add text in figure coords
figure	create or change active figure
fill	make filled polygons
fill_between	make filled polygons between two curves
findobj	recursively find all objects matching some criteria
gca	return the current axes
gcf	return the current figure
gci	get the current image, or None
getp	get a graphics property
grid	set whether gridding is on
hexbin	make a 2D hexagonal binning plot
hist	make a histogram
hold	set the axes hold state
ioff	turn interaction mode off
ion	turn interaction mode on
isinteractive	return True if interaction mode is on
imread	load image file into array
imsave	save array as an image file
imshow	plot image data
ishold	return the hold state of the current axes
legend	make an axes legend
locator_params	adjust parameters used in locating axis ticks
loglog	a log log plot
matshow	display a matrix in a new figure preserving aspect
margins	set margins used in autoscaling
pcolor	make a pseudocolor plot
pcolormesh	make a pseudocolor plot using a quadrilateral mesh
pie	make a pie chart
plot	make a line plot
plot_date	plot dates
plotfile	plot column data from an ASCII tab/space/comma delimited file
pie	pie charts
polar	make a polar plot on a PolarAxes
psd	make a plot of power spectral density
quiver	make a direction field (arrows) plot
rc	control the default params
rgrids	customize the radial grids and labels for polar
savefig	save the current figure
scatter	make a scatter plot
setp	set a graphics property
semilogx	log x axis
semilogy	log y axis
show	show the figures
specgram	a spectrogram plot
spy	plot sparsity pattern using markers or image
stem	make a stem plot
subplot	make a subplot (numrows, numcols, axesnum)
subplots_adjust	change the params controlling the subplot positions of current figure
subplot_tool	launch the subplot configuration tool
suptitle	add a figure title
table	add a table to the plot
text	add some text at location x,y to the current axes
thetagrids	customize the radial theta grids and labels for polar
tick_params	control the appearance of ticks and tick labels
ticklabel_format	control the format of tick labels
title	add a title to the current axes
tricontour	make a contour plot on a triangular grid
tricontourf	make a filled contour plot on a triangular grid
tripcolor	make a pseudocolor plot on a triangular grid
triplot	plot a triangular grid
xcorr	plot the autocorrelation function of x and y
xlim	set/get the xlimits
ylim	set/get the ylimits
xticks	set/get the xticks
yticks	set/get the yticks
xlabel	add an xlabel to the current axes
ylabel	add a ylabel to the current axes
autumn	set the default colormap to autumn
bone	set the default colormap to bone
cool	set the default colormap to cool
copper	set the default colormap to copper
flag	set the default colormap to flag
gray	set the default colormap to gray
hot	set the default colormap to hot
hsv	set the default colormap to hsv
jet	set the default colormap to jet
pink	set the default colormap to pink
prism	set the default colormap to prism
spring	set the default colormap to spring
summer	set the default colormap to summer
winter	set the default colormap to winter
spectral	set the default colormap to spectral