Beginner·3 graded tests

Variables and Types

Python assigns a type to every value. The type determines which operations are valid on that value and how those operations behave. A variable is a name bound to a value; the name itself carries no type, and rebinding a name to a value of a different type is permitted.

The four built-in types used most often are int for whole numbers, float for real numbers, str for text, and bool for truth values.

[1]python

out1ms

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

Conversion between types

The type names double as conversion functions. int("120") produces the integer 120; str(120) produces the string "120". Conversion is explicit in Python: adding a string to an integer raises TypeError rather than guessing what was intended.

[2]python

out1ms

121
1.02
42 reads

Conversion fails when the text does not represent a valid value of the target type. int("twelve") raises ValueError, as does int("3.5"), because int accepts only whole-number string representations. float("3.5") succeeds. This asymmetry matters when validating input read from a file, where both forms occur.

[3]python

out1ms

120 -> 120
0.51 -> ValueError: invalid literal for int() with base 10: '0.51'
twelve -> ValueError: invalid literal for int() with base 10: 'twelve'

Numeric operators

Division with / always produces a float, even when the operands divide exactly: 6 / 3 is 2.0, not 2. Floor division with // discards the fractional part and returns an integer when both operands are integers. The modulo operator % returns the remainder.

[4]python

out1ms

3.5 3 1
2.0 <class 'float'>

These three operators together handle position arithmetic. A coding-sequence position divided by three gives the codon number; the remainder gives the offset within that codon. Note that the results below are zero-based, which is the convention Python uses throughout.

[5]python

out1ms

position 0: codon 0, offset 0
position 1: codon 0, offset 1
position 2: codon 0, offset 2
position 3: codon 1, offset 0
position 4: codon 1, offset 1
position 5: codon 1, offset 2

Floating-point precision

float values are stored in binary and cannot represent every decimal fraction exactly. 0.1 + 0.2 evaluates to a value marginally greater than 0.3, so comparing floats with == is unreliable. Compare with a tolerance instead, or round both sides to a fixed number of decimal places.

[6]python

out1ms

0.30000000000000004
False
True

Task

Write describe(value). It receives a string and must return:

"int <n>" if the string converts to a whole number.
"float <f>" if it converts to a real number but not a whole one.
"str <value>" otherwise.

Attempt the int conversion before the float conversion, because int("3") and float("3") both succeed and the integer classification takes precedence.

exerciseEvery submission runs independently in a fresh namespace.

Test cases · 3

#	via	input	expected stdout
1	stdin	120	int 120
2	stdin	0.51	float 0.51
3	stdin	TP53	str TP53

Hints · 2

01Hint

int('3.5') raises ValueError, but float('3.5') does not.

02Hint

Wrap each conversion in its own try/except ValueError block.