**A training to acquire strong basis in Python to use it efficiently**
Pierre Augier (LEGI), Cyrille Bonamy (LEGI), Eric Maldonado (Irstea), Franck Thollard (ISTerre), Christophe Picard (LJK), Loïc Huder (ISTerre)
# Standard types and basic statements
%% Cell type:markdown id: tags:
## Function calls
There are [built-in functions](https://docs.python.org/3/library/functions.html) and the developers can of course define other functions. To call a function:
Remark: `int` in Python 3 are impressive! No limit!
See https://docs.python.org/3.1/whatsnew/3.0.html#integers
%% Cell type:markdown id: tags:
## Arithmetic operations
%% Cell type:code id: tags:
``` python
print(10+3)
print(10-3)
print(10*3)
print(10/3)# float division
print(10//3)# integer division
print(10%3)
```
%%%% Output: stream
13
7
30
3.3333333333333335
3
1
%% Cell type:markdown id: tags:
### `bool` (booleans)
%% Cell type:code id: tags:
``` python
b=bool("1")
b=False
b=True
```
%% Cell type:markdown id: tags:
##### Comparison operations (bool)
-`==` equal
-`!=` différent
-`<` inferior
-`<=` inferior or equal
-`>` superior
-`>=` superior or equal
%% Cell type:markdown id: tags:
##### Keyword `is`: check identity
%% Cell type:code id: tags:
``` python
a=None
print(aisNone)
print(aisnotNone)
```
%%%% Output: stream
True
False
%% Cell type:markdown id: tags:
##### Keywords `and` and `or`
%% Cell type:code id: tags:
``` python
TrueandTrue
```
%%%% Output: execute_result
True
%% Cell type:code id: tags:
``` python
TrueandFalse
```
%%%% Output: execute_result
False
%% Cell type:code id: tags:
``` python
FalseandFalse
```
%%%% Output: execute_result
False
%% Cell type:code id: tags:
``` python
TrueorTrue
```
%%%% Output: execute_result
True
%% Cell type:code id: tags:
``` python
TrueorFalse
```
%%%% Output: execute_result
True
%% Cell type:code id: tags:
``` python
FalseorFalse
```
%%%% Output: execute_result
False
%% Cell type:markdown id: tags:
### `float` (real, double precision) and `complex`
%% Cell type:code id: tags:
``` python
# float
a=float("1")
a=1.234
a=1e2
a=-1e-2
a=0.2
```
%% Cell type:code id: tags:
``` python
# complex (2 floats)
c=complex("1")
c=1+2j
print(c,c.real,c.imag)
```
%%%% Output: stream
(1+2j) 1.0 2.0
%% Cell type:markdown id: tags:
Remark: notation `var_name.attr_name` to access to an attribute of an object.
%% Cell type:markdown id: tags:
## Warning about floating-point arithmetic and numerical errors!
%% Cell type:code id: tags:
``` python
b=1e16
c=1.2+b
d=c-b
print(d)
```
%%%% Output: stream
2.0
%% Cell type:markdown id: tags:
Very general issue (not Python):
see https://en.wikipedia.org/wiki/Floating-point_arithmetic
%% Cell type:markdown id: tags:
### Standard type `str`
%% Cell type:code id: tags:
``` python
s="hello"
s="hello"
s=(
"How is it possible to write a very very "
"very long string with lines limited to 79 characters?"
)
s="""Strings on
more thand
one line.
"""
print(s)
```
%%%% Output: stream
Strings on
more thand
one line.
%% Cell type:markdown id: tags:
Warning: big difference between Python 2 and Python 3. In Python 3, `str` are unicode and there is another type `bytes`.
%% Cell type:markdown id: tags:
#### Methods of the type `str`
Objects of built-in types have methods associated with their type (object oriented programming). The built-in function `dir` returns a list of name of the attributes. For a string, these attributes are python system attributes (with double-underscores) and several public methods:
Very general, can be used on all sequences as `str`, `list`, etc... Not simple for beginners but very powerfull (see [here](http://stackoverflow.com/questions/509211/explain-pythons-slice-notation) and [here](http://avilpage.com/2015/03/a-slice-of-python-intelligence-behind.html)).
Python indexes and slices for a six-element str. Indexes enumerate the elements, slices enumerate the spaces between the elements.
```text
Index from rear: -6 -5 -4 -3 -2 -1
Index from front: 0 1 2 3 4 5
+---+---+---+---+---+---+
| a | b | c | d | e | f |
+---+---+---+---+---+---+
Slice from front: 0 1 2 3 4 5 6
Slice from rear: -6 -5 -4 -3 -2 -1 None
```
%% Cell type:code id: tags:
``` python
s = "abcdef"
# s[start:stop:step]
s[2:6:2]
```
%%%% Output: execute_result
'ce'
%% Cell type:code id: tags:
``` python
# s[start:stop]
s[2:6]
```
%%%% Output: execute_result
'cdef'
%% Cell type:code id: tags:
``` python
# s[start:]
s[1:]
```
%%%% Output: execute_result
'bcdef'
%% Cell type:code id: tags:
``` python
# s[:stop]
s[:2]
```
%%%% Output: execute_result
'ab'
%% Cell type:code id: tags:
``` python
# step = -1: goes through the string in reverse order
s[::-1]
```
%%%% Output: execute_result
'fedcba'
%% Cell type:markdown id: tags:
### standard type `list`
A list is a **mutable sequence of (possibly inhomogeneous) elements**.
%% Cell type:code id: tags:
``` python
type([0, "a"])
```
%%%% Output: execute_result
list
%% Cell type:code id: tags:
``` python
# create an empty list
l = []
# fill the list (with the function append)
l.append("2")
# fill the list (with the function extend)
l.extend([6, 3.0])
print(l)
```
%%%% Output: stream
['2', 6, 3.0]
%% Cell type:code id: tags:
``` python
# concatenate lists with the operator +
print(l + ["hello", 3])
```
%%%% Output: stream
['2', 6, 3.0, 'hello', 3]
%% Cell type:code id: tags:
``` python
# get values
print(l[0], l[2], l[-2])
# slicing
print(l[0:2])
```
%%%% Output: stream
2 3.0 6
['2', 6]
%% Cell type:markdown id: tags:
### standard type `tuple`
A tuple is a **immutable sequence of (possibly inhomogeneous) elements**.
Remark: when you need a sequence that won't be modified, tuple is usually more efficient than list.
%% Cell type:code id: tags:
``` python
t = 0, "a", 1.2
t1 = (5, "hello")
t2 = tuple([1.1, 2])
type(t)
```
%%%% Output: execute_result
tuple
%% Cell type:code id: tags:
``` python
t[1] # indexing
```
%%%% Output: execute_result
'a'
%% Cell type:code id: tags:
``` python
t[1:] # slicing
```
%%%% Output: execute_result
('a', 1.2)
%% Cell type:code id: tags:
``` python
a, b = t1 # tuple assigment
print(b)
```
%%%% Output: stream
hello
%% Cell type:markdown id: tags:
## Mutable and immutable objects
The objects of type `str`, `int`, `float`, `bool` are immutable. They can not be modified. Of course, a name that points towards an integer can point towards a different integer.
%% Cell type:code id: tags:
``` python
i = 1
i = i + 2 # (or i += 2)
print(i)
i = 10
print(i)
```
%%%% Output: stream
3
10
%% Cell type:markdown id: tags:
Here, the objects `1` and `3` have not been modified.
%% Cell type:markdown id: tags:
## Mutable and immutable objects
An object of type `list` is mutable:
%% Cell type:code id: tags:
``` python
l = [0, 5]
print(l)
l.append("hello")
print(l)
```
%%%% Output: stream
[0, 5]
[0, 5, 'hello']
%% Cell type:markdown id: tags:
Here, the object list tagged by the name `l` has been modified inplace.
%% Cell type:markdown id: tags:
## References and `del` keyword
`del` removes a reference. If an object in not binded to any names, Python can delete it from its internal memory.
%% Cell type:code id: tags:
``` python
l = ["a", "b"]
del l[1]
print(l)
```
%%%% Output: stream
['a']
%% Cell type:markdown id: tags:
### More on slicing (shallow copy)
#### Assigment to mutable object
%% Cell type:code id: tags:
``` python
l = [0, 1, 2, 3, 4, 5]
l1 = l # assigment to a new name l1 (no copy of the object).
# the names l and l1 points towards the same object.
l1.append("a")
print(l1)
print(l)
```
%%%% Output: stream
[0, 1, 2, 3, 4, 5, 'a']
[0, 1, 2, 3, 4, 5, 'a']
%% Cell type:markdown id: tags:
#### Shallow copy
%% Cell type:code id: tags:
``` python
l = [0, 1, 2, 3, 4, 5]
l1 = l[:] # shallow copy of l
l1.append("a")
print(l1)
print(l)
```
%%%% Output: stream
[0, 1, 2, 3, 4, 5, 'a']
[0, 1, 2, 3, 4, 5]
%% Cell type:markdown id: tags:
### More examples of slicing
Other examples of slices for a six-element list. Indexes enumerate the elements, slices enumerate the spaces between the elements.
%% Cell type:code id: tags:
``` python
a = [0, 1, 2, 3, 4, 5]
all(
[
len(a) == 6,
a[1:] == [1, 2, 3, 4, 5],
a[:5] == [0, 1, 2, 3, 4],
a[0] == 0,
a[:-2] == [0, 1, 2, 3],
a[5] == 5,
a[1:2] == [1],
a[-1] == 5,
a[1:-1] == [1, 2, 3, 4],
a[-2] == 4,
]
)
```
%%%% Output: execute_result
True
%% Cell type:markdown id: tags:
### Do it yourself
Suppose we have the string containing header line.
%% Cell type:code id: tags:
``` python
s = " wind;temperature;;pressure "
```
%% Cell type:markdown id: tags:
1. Remove leading and ending blanks (with `str.strip`)
2. Extract the first field (with `str.find` and slicing)
3. Extract the last field (with `str.rfind` and slicing)
4. Check for empty field (with the `";;" in ...` pattern)
5. Remove empty field (with `str.replace`)
For each task, do not hesitate to print the help of the method by writing `s.strip?` in IPython.
%% Cell type:markdown id: tags:
#### A possible solution:
%% Cell type:code id: tags:
``` python
s = " wind;temperature;;pressure "
# remove leading blanks
s = s.strip()
f"--{s}--"
```
%%%% Output: execute_result
'--wind;temperature;;pressure--'
%% Cell type:code id: tags:
``` python
# extract the first field
idx = s.find(";")
s0 = s[0:idx]
s0
```
%%%% Output: execute_result
'wind'
%% Cell type:code id: tags:
``` python
# extract the second field
idx1 = s.find(";", idx + 1) # start the search after the first ";"
s1 = s[idx + 1 : idx1]
s1
```
%%%% Output: execute_result
'temperature'
%% Cell type:code id: tags:
``` python
# extract the last field
idx2 = s.rfind(";")
s2 = s[idx2 + 1 :]
s2
```
%%%% Output: execute_result
'pressure'
%% Cell type:code id: tags:
``` python
# check presence of ";;"
";;" in s
```
%%%% Output: execute_result
True
%% Cell type:code id: tags:
``` python
# remove empty field
s_no_empty = s.replace(";;", ";")
s_no_empty
```
%%%% Output: execute_result
'wind;temperature;pressure'
%% Cell type:markdown id: tags:
### The function `range`
The function returns a range object:
%% Cell type:code id: tags:
``` python
# start, stop, step
range(1, 8, 2)
```
%%%% Output: execute_result
range(1, 8, 2)
%% Cell type:markdown id: tags:
We can make a list with the range object:
%% Cell type:code id: tags:
``` python
# start, stop, step
list(range(1, 8, 2))
```
%%%% Output: execute_result
[1, 3, 5, 7]
%% Cell type:code id: tags:
``` python
# start, stop (step=1)
list(range(2, 8))
```
%%%% Output: execute_result
[2, 3, 4, 5, 6, 7]
%% Cell type:code id: tags:
``` python
# stop argument (start=0, step=1)
list(range(8))
```
%%%% Output: execute_result
[0, 1, 2, 3, 4, 5, 6, 7]
%% Cell type:markdown id: tags:
### Do it yourself
Build a list of odd numbers in decreasing order.
%% Cell type:markdown id: tags:
## Conditions: `if`, `elif`, `else`
```python
if expression:
statement(s)
else:
statement(s)
```
The statement contains the block of code that executes if the conditional expression in the if statement resolves to 1 or a TRUE value.
