Checking a Type with type()

Part I · Module 2 · Chapter 2.10 · Checking a Type with type()

1. Asking Python What It Has

print(type(4210))

<class 'int'>

type is a name Python provides, used the same way print is: write the name, then brackets, then the value you are asking about.

Here it is applied to each of the value kinds from this module:

print(type(4210))
print(type(182.4))
print(type("axi_smoke"))
print(type(True))
print(type(None))
print(type(3 + 4j))

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

Six values, six types. The part you care about is the word in quotes: int, float, str, bool, NoneType, complex.

2. Why It Says "class"

The obvious question, answered plainly.

Python organises types through a general system in which a type is described by something called a class. <class 'int'> is Python telling you the type of this value is the one called int.

You do not need that machinery yet. Classes are taught properly in Module 16 — Object-Oriented Programming, at a point where the rest of the language makes them worth having.

3. Settling the Earlier Questions

Three chapters made a claim you could not check. Now you can.

4210 and "4210"

print(type(4210))
print(type("4210"))

<class 'int'>
<class 'str'>

Chapter 2.9 §3 said these were different values that print identically. Here is the proof.

True and "True"

print(type(True))
print(type("True"))

<class 'bool'>
<class 'str'>

None and "None"

print(type(None))
print(type("None"))

<class 'NoneType'>
<class 'str'>

In every case the second value is a str. Quotes make text, and text is never the thing it spells — now demonstrated rather than asserted.

4. Checking a Name

type works on a name just as well as on a written-out value, because a name refers to a value:

seed = 4210
seed_text = "4210"

print(type(seed))
print(type(seed_text))

<class 'int'>
<class 'str'>

This is the form you will actually use, because in a real script the value came from somewhere else and you cannot see it written down.

5. Using It as a Diagnostic

This is what the tool is for.

Something in a script does not behave the way you expect. A number will not add up; a comparison gives the wrong answer; a value that should be missing is not. Very often the cause is that a value is not the type you assumed.

# Read from a file, a report or a command line.
# It looks like a number. Is it?
value = "12"

print(value)
print(type(value))

12
<class 'str'>

print showed 12 and told you nothing useful. type showed that it is text. That is the answer, and it took one line.

6. What to Do About It

Knowing a value is the wrong type is half the problem. The other half — converting it — is the next chapter.

For now: type diagnoses, it does not fix. It changes nothing about the value; it only reports.

7. Predict, Run, Observe

Predict each line of output — the full <class '...'> form — then run the file.

print(type(0))
print(type(0.0))
print(type("0"))
print(type(False))
print(type(""))
print(type(None))

<class 'int'>
<class 'float'>
<class 'str'>
<class 'bool'>
<class 'str'>
<class 'NoneType'>

Two to check yourself on. 0.0 is a float even though the value is whole — Chapter 2.6 §2, the decimal point decides. And "" is a str even though it contains nothing — empty text is still text, which is why Chapter 2.9 §6 insisted it is not None.

8. Exercise

Exercise 1 — predict all six. Without running, write down the type of each: 500, "500", 5.0, True, "True", None. Then check every one.

Exercise 2 — diagnose it. Here is a fragment where something is wrong:

error_count = "0"
print(error_count)

The output looks correct. Add one line that reveals the problem, and say in a sentence what a script trying to use this value would find.

Exercise 3 — name or value. Show that type gives the same answer whether you pass a written-out value or a name referring to it. Use a seed as the example.

Exercise 4 — the odd one out. Of these five, four share a type. Which is the odd one, and what is each type?

print(type("PASS"))
print(type("4210"))
print(type(4210))
print(type("True"))
print(type(""))

9. Summary

• type(value) reports the kind of a value. Use it like print — name, brackets, value.
• Read <class 'int'> as "this value has type int". The word class comes from machinery taught in Module 16 and is not needed now.
• The six types in this module: int, float, str, bool, NoneType, complex.
• It works on names as well as written-out values, which is the form real scripts use.
• It is a diagnostic: when something behaves oddly, check the type before anything else. print shows what a value looks like; type shows what it is.
• It reports, it does not convert. Changing a type is the next chapter.

You can now find out that a value is text when you needed a number. The next chapter is about what to do when that happens — and it is the chapter most beginner log parsers eventually fail on.