Python Truthiness
An empty text is not the value False, but a condition treats it as false. bool("") asks that question and writes False. bool("lida") writes True, because the text has characters. bool answers with True or False, the two values from the previous page. It does not change what is stored.
print(bool(""))
print(bool("lida"))
The first line is False. The second line is True. The quotes in "" have nothing between them.
Zero is not the text 0
bool(0) writes False. The number 0 is empty of any count, so a condition treats it as false. bool("0") writes True. That one is text: a character sits between the quotes, so it is not empty. bool(2) writes True. Any number other than 0 is treated as true. This page does not turn text into a number.
print(bool(0))
print(bool("0"))
print(bool(2))
The three lines are False, then True, then True.
A space is a character
bool(" ") writes True. One space sits between the quotes. The text is not empty, so a condition treats it as true. Only quotes with nothing between them are the empty text.
A name follows the same rule
guest = "" stores an empty text. bool(guest) writes False. A later line, guest = "lida", replaces it. bool(guest) then writes True. The name is not what matters. What it holds matters.
guest = ""
print(bool(guest))
guest = "lida"
print(bool(guest))
The first print is False. The second print is True.
The next page is a missing value
This page does not compare two numbers, choose a branch, or join two conditions. The next page is a value that means nothing is stored. A condition does not treat that one as true either. You meet it there, not here.
Say these yourself
- bool("") writes False. bool("lida") writes True.
- bool(0) writes False. bool("0") writes True. bool(2) writes True.
- bool(" ") writes True, because a space is a character.
- The next page is a value that means nothing is stored.
Easy to misread
- Expecting bool("") to write an empty line
- Treating the text "0" like the number 0
- Treating one space as empty
- Thinking bool changes the stored text
- Waiting for a branch or a comparison on this page
Keep
bool asks whether a condition would treat a value as true, and answers True or False.
Empty text and the number 0 are treated as false.
"lida", the text "0", the number 2, and a single space are treated as true.
A later line can replace what a name holds, and bool follows that.
The next page is a value that means nothing is stored.
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