Introduction#

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History#

  • Project initiated by Guido Von Rossum in 1990

  • Interpreted language written in C.

  • Widely used in all domains (Web, Data Science, Scientific Computation).

  • This is a high level language with a simple syntax.

  • Python types are numerously and powerful.

  • Bind Python with other languages is easy.

  • You can perform a lot of operations with very few lines.

  • Available on all platforms Unix, Windows, Mac OS X…

  • Many libraries offer Python bindings.

  • Python 2 is retired use only Python 3

Python distributions#

Python packages are available with all linux distributions but you can get standalone bundles:

Performances#

Python is not fast… but:

  • Sometimes it is.

  • Most of operations are optimized.

  • Package like numpy can reduce the CPU time.

  • With Python you can save time to achieve your project.

Some advices:

  • Write your program with Python language.

  • If it is fast enough, be happy.

  • After profiling, optimize costly parts of your code.

“Premature optimization is the root of all evil” (Donald Knuth 1974)

Jupyter - Start The Notebook#

Open the notebook

git clone https:://github.com/pnavaro/python-notebooks
cd python-notebooks/notebooks
jupyter notebook

You should see the notebook open in your browser. If not, go to http://localhost:8888

The Jupyter Notebook is an interactive environment for writing and running code. The notebook is capable of running code in a wide range of languages. However, each notebook is associated with Python3 kernel.

Code cells allow you to enter and run code#

Make a copy of this notebook by using the File menu.

Run a code cell using Shift-Enter or pressing the button in the toolbar above:

There are two other keyboard shortcuts for running code:

  • Alt-Enter runs the current cell and inserts a new one below.

  • Ctrl-Enter run the current cell and enters command mode.

Managing the Kernel#

Code is run in a separate process called the Kernel. The Kernel can be interrupted or restarted. Try running the following cell and then hit the button in the toolbar above.

The “Cell” menu has a number of menu items for running code in different ways. These includes:

  • Run and Select Below

  • Run and Insert Below

  • Run All

  • Run All Above

  • Run All Below

Restarting the kernels#

The kernel maintains the state of a notebook’s computations. You can reset this state by restarting the kernel. This is done by clicking on the in the toolbar above.

Check the documentation.

First program#

  • Print out the string “Hello world!” and its type.

  • Print out the value of a variable set to 6625 and its type.

s = "Hello World!"
print(type(s),s)

<class 'str'> Hello World!

a = 6625
print(type(a),a)

<class 'int'> 6625

# a+s

Execute using python#

%%file hello.py

s = "Hello World!"
print(type(s),s)
a = 6625
print(type(a),a)

Writing hello.py

$ python3 hello.py
<class 'str'> Hello World!
<class 'int'> 6625

Execute with ipython#

(my-env) $ ipython
Python 3.6.3 | packaged by conda-forge | (default, Nov  4 2017, 10:13:32)
Type 'copyright', 'credits' or 'license' for more information
IPython 6.2.1 -- An enhanced Interactive Python. Type '?' for help.

In [1]: run hello.py
<class 'str'> Hello World!
<class 'int'> 6625

%run hello.py

<class 'str'> Hello World!
<class 'int'> 6625

Python Types#

  • Most of Python types are classes, typing is dynamic.

  • ; symbol can be used to split two Python commands on the same line.

s = int(2010); print(type(s))
s = 3.14; print(type(s))
s = True; print(type(s))
s = None; print(type(s))
s = 1.0j; print(type(s))
s = type(type(s)); print(type(s))

<class 'int'>
<class 'float'>
<class 'bool'>
<class 'NoneType'>
<class 'complex'>
<class 'type'>

Calculate with Python#

x = 45        # This is a comment!
x += 2        # equivalent to x = x + 2
print(x, x > 45)

47 True

y = 2.5
print("x+y=",x+y, type(x+y))  # Add float to integer, result will be a float

x+y= 49.5 <class 'float'>

print(x*10/y)   # true division returns a float
print(x*10//3)  # floor division discards the fractional part

188.0
156

print( x % 8) # the % operator returns the remainder of the division

7

print( f" x = {x:05d} ") # You can use C format rules to improve print output

 x = 00047

Multiple Assignment#

  • Variables can simultaneously get new values.

  • Expressions on the right-hand side are all evaluated first before assignments take place.

  • The right-hand side expressions are evaluated from the left to the right.

  • Use it very carefully

a = b = c = 1
print(a, b, c) 

1 1 1

a, b, c = 1, 2, 3
print (a, b, c)

1 2 3

a, c = c, a     # Nice way to permute values
print (a, b, c) 

3 2 1

a < b < c, a > b > c

(False, True)

input Function#

  • Value returned by input is a string.

  • You must cast input call to get the type you want.

name = input("Please enter your name: ")
x = int(input("Please enter an integer: "))
L = list(input("Please enter 3 integers "))

Copy-pase code above in three different cells and print returned values.