Why does typing a variable (or expression) print the value to stdout?

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?

edited Feb 25 at 12:41

Peter Mortensen

13.8k1987113

asked Feb 25 at 4:19

Chayan Ghosh

1507

add a comment |

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?

edited Feb 25 at 12:41

Peter Mortensen

13.8k1987113

asked Feb 25 at 4:19

Chayan Ghosh

1507

add a comment |

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?

edited Feb 25 at 12:41

Peter Mortensen

13.8k1987113

asked Feb 25 at 4:19

Chayan Ghosh

1507

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?

python ipython read-eval-print-loop python-interactive

edited Feb 25 at 12:41

Peter Mortensen

13.8k1987113

asked Feb 25 at 4:19

Chayan Ghosh

1507

edited Feb 25 at 12:41

Peter Mortensen

13.8k1987113

asked Feb 25 at 4:19

Chayan Ghosh

1507

edited Feb 25 at 12:41

Peter Mortensen

13.8k1987113

edited Feb 25 at 12:41

Peter Mortensen

13.8k1987113

edited Feb 25 at 12:41

Peter Mortensen

13.8k1987113

asked Feb 25 at 4:19

Chayan Ghosh

1507

asked Feb 25 at 4:19

Chayan Ghosh

1507

asked Feb 25 at 4:19

Chayan Ghosh

1507

add a comment |

2 Answers
2

active

oldest

votes

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.

edited Feb 25 at 13:52

answered Feb 25 at 4:33

kojiro

54.2k1390142

add a comment |

That's how all interpreters work. They don't need any print, but one thing, and without print they do the repr of everything, and print doesn't, example:

>>> 'blah'

'blah'

>>> print('blah')

blah

>>>

Look at the quotes.

Also see this:

>>> print(repr('blah'))

'blah'

>>>

repr does the same.

edited Feb 25 at 12:43

Peter Mortensen

13.8k1987113

answered Feb 25 at 4:21

U9-Forward

17.5k51743

1

any comment on the IPython behavior?

– Chayan Ghosh
Feb 25 at 4:25

2

Let's say CPython in interactive mode works like that.

– Klaus D.
Feb 25 at 4:25

add a comment |

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2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

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.

edited Feb 25 at 13:52

answered Feb 25 at 4:33

kojiro

54.2k1390142

add a comment |

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.

edited Feb 25 at 13:52

answered Feb 25 at 4:33

kojiro

54.2k1390142

add a comment |

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.

edited Feb 25 at 13:52

answered Feb 25 at 4:33

kojiro

54.2k1390142

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.

edited Feb 25 at 13:52

answered Feb 25 at 4:33

kojiro

54.2k1390142

edited Feb 25 at 13:52

answered Feb 25 at 4:33

kojiro

54.2k1390142

answered Feb 25 at 4:33

kojiro

54.2k1390142

answered Feb 25 at 4:33

kojiro

54.2k1390142

add a comment |

That's how all interpreters work. They don't need any print, but one thing, and without print they do the repr of everything, and print doesn't, example:

>>> 'blah'

'blah'

>>> print('blah')

blah

>>>

Look at the quotes.

Also see this:

>>> print(repr('blah'))

'blah'

>>>

repr does the same.

edited Feb 25 at 12:43

Peter Mortensen

13.8k1987113

answered Feb 25 at 4:21

U9-Forward

17.5k51743

1

any comment on the IPython behavior?

– Chayan Ghosh
Feb 25 at 4:25

2

Let's say CPython in interactive mode works like that.

– Klaus D.
Feb 25 at 4:25

add a comment |

That's how all interpreters work. They don't need any print, but one thing, and without print they do the repr of everything, and print doesn't, example:

>>> 'blah'

'blah'

>>> print('blah')

blah

>>>

Look at the quotes.

Also see this:

>>> print(repr('blah'))

'blah'

>>>

repr does the same.

edited Feb 25 at 12:43

Peter Mortensen

13.8k1987113

answered Feb 25 at 4:21

U9-Forward

17.5k51743

1

any comment on the IPython behavior?

– Chayan Ghosh
Feb 25 at 4:25

2

Let's say CPython in interactive mode works like that.

– Klaus D.
Feb 25 at 4:25

add a comment |

That's how all interpreters work. They don't need any print, but one thing, and without print they do the repr of everything, and print doesn't, example:

>>> 'blah'

'blah'

>>> print('blah')

blah

>>>

Look at the quotes.

Also see this:

>>> print(repr('blah'))

'blah'

>>>

repr does the same.

edited Feb 25 at 12:43

Peter Mortensen

13.8k1987113

answered Feb 25 at 4:21

U9-Forward

17.5k51743

That's how all interpreters work. They don't need any print, but one thing, and without print they do the repr of everything, and print doesn't, example:

>>> 'blah'

'blah'

>>> print('blah')

blah

>>>

Look at the quotes.

Also see this:

>>> print(repr('blah'))

'blah'

>>>

repr does the same.

edited Feb 25 at 12:43

Peter Mortensen

13.8k1987113

answered Feb 25 at 4:21

U9-Forward

17.5k51743

edited Feb 25 at 12:43

Peter Mortensen

13.8k1987113

edited Feb 25 at 12:43

Peter Mortensen

13.8k1987113

edited Feb 25 at 12:43

Peter Mortensen

13.8k1987113

answered Feb 25 at 4:21

U9-Forward

17.5k51743

answered Feb 25 at 4:21

U9-Forward

17.5k51743

answered Feb 25 at 4:21

U9-Forward

17.5k51743

1

any comment on the IPython behavior?

– Chayan Ghosh
Feb 25 at 4:25

2

Let's say CPython in interactive mode works like that.

– Klaus D.
Feb 25 at 4:25

add a comment |

1

any comment on the IPython behavior?

– Chayan Ghosh
Feb 25 at 4:25

2

Let's say CPython in interactive mode works like that.

– Klaus D.
Feb 25 at 4:25

any comment on the IPython behavior?

– Chayan Ghosh
Feb 25 at 4:25

Let's say CPython in interactive mode works like that.

– Klaus D.
Feb 25 at 4:25

add a comment |

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