Take this example:
>>> 5+10
15
>>> a = 5 + 10
>>> a
15
How and why does Python do this without an explicit print statement?
If I do the same thing in an IPython cell, only the last such value is actually printed on stdout in this way:
In[1]: 5+10
1
Out[1]: 1
Why does this happen?
When Python is in "interactive" mode, it enables certain behaviors it doesn't have in non-interactive mode. For example, sys.displayhook
, originally specified in PEP 217.
If value is not None, this function prints it to sys.stdout, and saves it in
__builtin__._
.sys.displayhook is called on the result of evaluating an expression entered in an interactive Python session.
You can modify this behavior:
>>> import sys
>>> def shook(expr):
... print(f'can haz {expr}?')
...
>>> sys.displayhook = shook
>>> 123
can haz 123?
>>> False
can haz False?
>>> None
can haz None?
And also set it back to normal:
>>> sys.displayhook = sys.__displayhook__
>>> 3
3
In the default Python repl, sys.displayhook
is
>>> import sys;
>>> sys.displayhook
<built-in function displayhook>
but in IPython it's
In [1]: import sys
In [2]: sys.displayhook
Out[2]: <IPython.terminal.prompts.RichPromptDisplayHook at 0x7f630717fa58>
So that's why you see different behavior between Python and IPython.
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