Programming languages

Problem:

You want to run a Python script, e.g. Pronterface, but you see an error message like this:

wxPython is not installed. This program requires wxPython to run.
Traceback (most recent call last):
  File "./pronterface.py", line 23, in <module>
    import wx  # NOQA
ImportError: No module named wx

Solution:

sudo apt install python-wxtools

The easiest way to parse a ISO8601 timestamp like 2019-05-29T19:50:55.463+02:00 is to install the maya library using e.g.

sudo pip3 install maya

In case you still use Python2, use pip instead of pip3.

After installing maya, use it like this:

import maya
result = maya.parse('2019-05-29T19:50:55.463+02:00').datetime()
# Result is a standard datetime object!
print(result)

This will print

2019-05-29 17:50:55.463000+00:00

Notice how the original timestamp from timezone +02:00 is automatically converted to a UTC datetime object.

maya can parse a lot of different formats e.g.

print(maya.parse('2019-05-29T19:50:55+02:00').datetime()) # No fractional seconds
print(maya.parse('2019-05-29T19:50:55').datetime()) # No timezone (UTC assumed)
print(maya.parse('2019-05-29 19:50:55').datetime()) # Not ISO8601
print(maya.parse('2019-05-29').datetime()) # 00:00:00 & UTC assumed

Problem:

You are trying to do a simple arithmetic operation on a NumPy array but you see an error message like

TypeError: Cannot cast ufunc subtract output from dtype('float64') to dtype('int64') with casting rule 'same_kind'

Solution:

You are trying to substract a float from an int64 array. This does not work with operators like += or -=

Example:

import numpy as np

data = np.asarray([1, 2, 3, 4], dtype=np.int64) # This is an int array!

print(data - 5) # This works
print(data - 5.0) # This works as well
# This raises: Cannot cast ufunc subtract output from dtype('float64') to dtype('int64') with casting rule 'same_kind'
data -= 5.0

Option 1 (preferred):

Use - instead of -=: Instead of data -= 5.0 use data = data - 5.0

Option 2:

Explicitly cast data to float (or the first dtype of your error message):

data = data.astype('float64')
# Now this works
data -= 5.0

This option is not preferred since doing it requires using the correct datatype. The first option works without regarding the actual datatype.

To iterate all days in a year using Python you can use this function snippet:

from collections import namedtuple
from calendar import monthrange

Date = namedtuple("Date", ["year", "month", "day"])

def all_dates_in_year(year=2019):
    for month in range(1, 13): # Month is always 1..12
        for day in range(1, monthrange(year, month)[1] + 1):
            yield Date(year, month, day)

Usage example:

for date in all_dates_in_year(2019):
    print(date)

This will print (excerpt):

Date(year=2019, month=1, day=1)
Date(year=2019, month=1, day=2)
Date(year=2019, month=1, day=3)
[...]
Date(year=2019, month=2, day=27)
Date(year=2019, month=2, day=28)
Date(year=2019, month=3, day=1)
[...]
Date(year=2019, month=12, day=29)
Date(year=2019, month=12, day=30)
Date(year=2019, month=12, day=31)

You can also install my Python3 library UliEngineering using sudo pip3 install -U UliEngineering and then use all_dates_in_year() by importing it like this:

from UliEngineering.Utils.Date import *

Also see How to get number of days in month in Python

If you want to find out how many days a month using Python use this snippet:

from calendar import monthrange

num_days = monthrange(2019, 2)[1] # num_days = 28

print(num_days) # Prints 28

The calendar module automatically takes into account leap years.

You can also use this function snippet:

from calendar import monthrange

def number_of_days_in_month(year=2019, month=2):
    return monthrange(year, month)[1]

# Usage example:
print(number_of_days_in_month(2019, 2)) # Prints 28

You can also install my Python3 library UliEngineering using sudo pip3 install -U UliEngineering and then use number_of_days_in_month() by importing it like this:

from UliEngineering.Utils.Date import *

NumPy’s datetime64 objects are represented as 64 bit integers (that’s what the 64 in the name means).

In order to find out what the resolution (e.g. us, ns etc) first install my Python 3 UliEngineering library using

sudo pip3 install -U UliEngineering

and then use datetime64_unit() from UliEngineering.Utils.NumPy like this:

from UliEngineering.Utils.NumPy import *

my_datetime = datetime64_now()
print(datetime64_resolution(my_datetime)) # Prints "us"

In order to get the unit (e.g. us, ms, ns, …) of a np.timedelta64 object like

my_timedelta = np.timedelta64(625, 'us') # Unit is 'us'

first install my Python 3 UliEngineering library using

sudo pip3 install -U UliEngineering

and then use timedelta64_unit() from UliEngineering.Utils.NumPy like this:

import numpy as np
from UliEngineering.Utils.NumPy import *

my_timedelta = np.timedelta64(625, 'us')
print(timedelta64_resolution(my_timedelta)) # Prints "us"

If you have a NumPy np.timedelta64 object like

import numpy as np
my_timedelta = np.timedelta64(625, 'us')

a common task is to convert it to an integer.

This is often as easy as

my_timedelta.astype(int) # = 625, type: np.int64

This will give you the number (which is always an integer!) stored in the np.timedelta64 object, however it will ignore the unit  (e.g. us i.e. microseconds in our example).

To get the unit of the timedelta, first install UliEngineering using sudo pip3 install -U UliEngineering which you can then use like this:

import numpy as np
from UliEngineering.Utils.NumPy import *

my_timedelta = np.timedelta64(625, 'us')
print(timedelta64_resolution(my_timedelta)) # Prints "us"

There are two basic ways to create a new np.timedelta64 object:

Construct directly:

import numpy as np
my_timedelta = np.timedelta64(625, 'us')

Note that the constructor will only work with ints, i.e. np.timedelta64(625.5, 'us') won’t work. See

Construct as a difference between two np.datetime64 objects:

See How to get current datetime as numpy datetime (np.datetime64) for more details on how to get the current datetime as np.datetime64.

For any two np.datetime64 instances, we can compute the difference using

from datetime import datetime
import numpy as np

# There will be a minimal time difference between those two
dt1 = np.datetime64(datetime.now())
dt2 = np.datetime64(datetime.now())

my_timedelta = dt2 - dt1
print(my_timedelta) # For me, this prints "16 microseconds"

The simplest way of getting the current time as np.datetime64 object is to combine datetime.datetime.now() and the np.datetime64() constructor:

from datetime import datetime
import numpy as np

dt_now = np.datetime64(datetime.now())

print(dt_now)

Or use this function for convenience:

from datetime import datetime
import numpy as np

def datetime64_now():
    return np.datetime64(datetime.now())

dt_now = datetime64_now()
print(dt_now)

datetime64_now() is also available on my library UliEngineering.
Install using sudo pip3 install -U UliEngineering, then use like this:

from UliEngineering.Utils.NumPy import datetime64_now

dt_now = datetime64_now()
print(dt_now)