TIL: Today I Learned 💡

This is a living collection of new findings, coding tips and gotchas that I found in my everyday work.

Data Science

Create train/split datasets with a mask

This code splits the dataset in 80% for training and 20% for testing:

mask = np.random.rand(len(df)) < 0.8
train = data_set[mask]
test = data_set[~mask]

Python

Reverse a string

When you pass step size -1 it reverses the string:

mystring[::-1]

and you can reverse every word in a string, maintaining the word order:

' '.join(s[::-1] for s in strs.split())

Create array/matrix with prefilled values

Creates 3x3 matrix where all elements equal to -1:

[[-1]*3]*3

[[-1, -1, -1], [-1, -1, -1], [-1, -1, -1]]

This approach should be avoided, because it creates list of references to the same element! Instead a list comprehension should be used:

[[-1]*3 for _ in range(3)]

Golang

defer call order

Since I normally use only one defer statement, I never thought of the execution order. Actually it’s LIFO:

defer print(2)
defer print(1)

So that will actually be printed as 12, even though the 2 statement is first. More practical case, if you want to close and then remove a file:

defer os.Remove(f.Name())
defer f.Close()

C#

Convert object to a template method type

If you want to convert object value to a type, specified in a template method, there’s a way to do it without if-s and case-es:

private T GetValue<T>(object objVal)
{
   var converter = TypeDescriptor.GetConverter(typeof(T));
   if (converter == null)
   {
      return default(T);
   }
   return (T)converter.ConvertFrom(objVal);
}

TypeScript

Type casting to an object with methods

If you have a class with attributes and methods and you want to type cast a JSON object to an instance of the class, then const obj = json as MyClass will only assign attributes, i.e. later calling something like obj.myMethod() will cause a no method error. To create a proper object you need to use Object.assign:

const obj = Object.assign(new MyClass(), json as MyClass)
obj.myMethod() // this will work now